diff --git a/src/testing/README.md b/src/testing/README.md
index 12b2a102d..dc19c66af 100644
--- a/src/testing/README.md
+++ b/src/testing/README.md
@@ -320,13 +320,13 @@ Here we're using the `setChildren()` api on the baseAssertion, and we're using t
 Assertion Template has the following api's:
 
 ```
-insertBefore(selector: string, children: DNode[]): AssertionTemplateResult;
-insertAfter(selector: string, children: DNode[]): AssertionTemplateResult;
-insertSiblings(selector: string, children: DNode[], type?: 'before' | 'after'): AssertionTemplateResult;
-append(selector: string, children: DNode[]): AssertionTemplateResult;
-prepend(selector: string, children: DNode[]): AssertionTemplateResult;
-replaceChildren(selector: string, children: DNode[]): AssertionTemplateResult;
-setChildren(selector: string, children: DNode[], type?: 'prepend' | 'replace' | 'append'): AssertionTemplateResult;
+insertBefore(selector: string, children: DNode[] | (() => DNode[])): AssertionTemplateResult;
+insertAfter(selector: string, children: DNode[] | (() => DNode[])): AssertionTemplateResult;
+insertSiblings(selector: string, children: DNode[] | (() => DNode[]), type?: 'before' | 'after'): AssertionTemplateResult;
+append(selector: string, children: DNode[] | (() => DNode[])): AssertionTemplateResult;
+prepend(selector: string, children: DNode[] | (() => DNode[])): AssertionTemplateResult;
+replaceChildren(selector: string, children: DNode[] | (() => DNode[])): AssertionTemplateResult;
+setChildren(selector: string, children: DNode[] | (() => DNode[]), type?: 'prepend' | 'replace' | 'append'): AssertionTemplateResult;
 setProperty(selector: string, property: string, value: any): AssertionTemplateResult;
 setProperties(selector: string, value: any | PropertiesComparatorFunction): AssertionTemplateResult;
 getChildren(selector: string): DNode[];
diff --git a/src/testing/assertionTemplate.ts b/src/testing/assertionTemplate.ts
index fb6042b90..ed36e31e5 100644
--- a/src/testing/assertionTemplate.ts
+++ b/src/testing/assertionTemplate.ts
@@ -6,15 +6,21 @@ import WidgetBase from '../core/WidgetBase';
 
 export type PropertiesComparatorFunction = (actualProperties: any) => any;
 
+export type TemplateChildren = DNode[] | (() => DNode[]);
+
 export interface AssertionTemplateResult {
 	(): DNode | DNode[];
-	append(selector: string, children: DNode[]): AssertionTemplateResult;
-	prepend(selector: string, children: DNode[]): AssertionTemplateResult;
-	replaceChildren(selector: string, children: DNode[]): AssertionTemplateResult;
-	insertBefore(selector: string, children: DNode[]): AssertionTemplateResult;
-	insertAfter(selector: string, children: DNode[]): AssertionTemplateResult;
-	insertSiblings(selector: string, children: DNode[], type?: 'before' | 'after'): AssertionTemplateResult;
-	setChildren(selector: string, children: DNode[], type?: 'prepend' | 'replace' | 'append'): AssertionTemplateResult;
+	append(selector: string, children: TemplateChildren): AssertionTemplateResult;
+	prepend(selector: string, children: TemplateChildren): AssertionTemplateResult;
+	replaceChildren(selector: string, children: TemplateChildren): AssertionTemplateResult;
+	insertBefore(selector: string, children: TemplateChildren): AssertionTemplateResult;
+	insertAfter(selector: string, children: TemplateChildren): AssertionTemplateResult;
+	insertSiblings(selector: string, children: TemplateChildren, type?: 'before' | 'after'): AssertionTemplateResult;
+	setChildren(
+		selector: string,
+		children: TemplateChildren,
+		type?: 'prepend' | 'replace' | 'append'
+	): AssertionTemplateResult;
 	setProperty(selector: string, property: string, value: any): AssertionTemplateResult;
 	setProperties(selector: string, value: any | PropertiesComparatorFunction): AssertionTemplateResult;
 	getChildren(selector: string): DNode[];
@@ -74,24 +80,32 @@ export function assertionTemplate(renderFunc: () => DNode | DNode[]) {
 			return render;
 		});
 	};
-	assertionTemplateResult.append = (selector: string, children: DNode[]) => {
+	assertionTemplateResult.append = (selector: string, children: TemplateChildren) => {
 		return assertionTemplateResult.setChildren(selector, children, 'append');
 	};
-	assertionTemplateResult.prepend = (selector: string, children: DNode[]) => {
+	assertionTemplateResult.prepend = (selector: string, children: TemplateChildren) => {
 		return assertionTemplateResult.setChildren(selector, children, 'prepend');
 	};
-	assertionTemplateResult.replaceChildren = (selector: string, children: DNode[]) => {
+	assertionTemplateResult.replaceChildren = (selector: string, children: TemplateChildren) => {
 		return assertionTemplateResult.setChildren(selector, children, 'replace');
 	};
 	assertionTemplateResult.setChildren = (
 		selector: string,
-		children: DNode[],
+		children: TemplateChildren,
 		type: 'prepend' | 'replace' | 'append' = 'replace'
 	) => {
+		if (Array.isArray(children)) {
+			console.warn(
+				'The array API (`children: DNode[]`) has been deprecated. Working with children should use a factory to avoid issues with mutation.'
+			);
+		}
 		return assertionTemplate(() => {
 			const render = renderFunc();
 			const node = findOne(render, selector);
 			node.children = node.children || [];
+			if (typeof children === 'function') {
+				children = children();
+			}
 			switch (type) {
 				case 'prepend':
 					node.children = [...children, ...node.children];
@@ -106,23 +120,31 @@ export function assertionTemplate(renderFunc: () => DNode | DNode[]) {
 			return render;
 		});
 	};
-	assertionTemplateResult.insertBefore = (selector: string, children: DNode[]) => {
+	assertionTemplateResult.insertBefore = (selector: string, children: TemplateChildren) => {
 		return assertionTemplateResult.insertSiblings(selector, children, 'before');
 	};
-	assertionTemplateResult.insertAfter = (selector: string, children: DNode[]) => {
+	assertionTemplateResult.insertAfter = (selector: string, children: TemplateChildren) => {
 		return assertionTemplateResult.insertSiblings(selector, children, 'after');
 	};
 	assertionTemplateResult.insertSiblings = (
 		selector: string,
-		children: DNode[],
+		children: TemplateChildren,
 		type: 'before' | 'after' = 'after'
 	) => {
+		if (Array.isArray(children)) {
+			console.warn(
+				'The array API (`children: DNode[]`) has been deprecated. Working with children should use a factory to avoid issues with mutation.'
+			);
+		}
 		return assertionTemplate(() => {
 			const render = renderFunc();
 			const node = findOne(render, selector);
 			const parent = (node as any).parent;
 			const index = parent.children.indexOf(node);
 			let newChildren = [...parent.children];
+			if (typeof children === 'function') {
+				children = children();
+			}
 			switch (type) {
 				case 'before':
 					newChildren.splice(index, 0, ...children);
diff --git a/tests/testing/unit/assertionTemplate.tsx b/tests/testing/unit/assertionTemplate.tsx
index edfe1c762..435b72a9d 100644
--- a/tests/testing/unit/assertionTemplate.tsx
+++ b/tests/testing/unit/assertionTemplate.tsx
@@ -1,5 +1,6 @@
-const { describe, it } = intern.getInterface('bdd');
+const { describe, it, after, afterEach } = intern.getInterface('bdd');
 const { assert } = intern.getPlugin('chai');
+import { stub } from 'sinon';
 
 import { harness } from '../../../src/testing/harness';
 import { WidgetBase } from '../../../src/core/WidgetBase';
@@ -78,7 +79,17 @@ const tsxAssertion = assertionTemplate(() => (
 	</div>
 ));
 
+let consoleStub = stub(console, 'warn');
+
 describe('assertionTemplate', () => {
+	afterEach(() => {
+		consoleStub.resetHistory();
+	});
+
+	after(() => {
+		consoleStub.restore();
+	});
+
 	it('can get a property', () => {
 		const classes = baseAssertion.getProperty('~root', 'classes');
 		assert.deepEqual(classes, ['root']);
@@ -134,36 +145,102 @@ describe('assertionTemplate', () => {
 	it('can set a child', () => {
 		const h = harness(() => w(MyWidget, { replaceChild: true }));
 		const childAssertion = baseAssertion.setChildren('~header', ['replace']);
+		assert.isTrue(consoleStub.calledOnce);
+		assert.strictEqual(
+			consoleStub.firstCall.args[0],
+			'The array API (`children: DNode[]`) has been deprecated. Working with children should use a factory to avoid issues with mutation.'
+		);
+		h.expect(childAssertion);
+	});
+
+	it('can set a child with a factory function', () => {
+		const h = harness(() => w(MyWidget, { replaceChild: true }));
+		const childAssertion = baseAssertion.setChildren('~header', () => ['replace']);
 		h.expect(childAssertion);
 	});
 
 	it('can set a child with replace', () => {
 		const h = harness(() => w(MyWidget, { replaceChild: true }));
 		const childAssertion = baseAssertion.replaceChildren('~header', ['replace']);
+		assert.isTrue(consoleStub.calledOnce);
+		assert.strictEqual(
+			consoleStub.firstCall.args[0],
+			'The array API (`children: DNode[]`) has been deprecated. Working with children should use a factory to avoid issues with mutation.'
+		);
+		h.expect(childAssertion);
+	});
+
+	it('can set a child with replace with a factory function', () => {
+		const h = harness(() => w(MyWidget, { replaceChild: true }));
+		const childAssertion = baseAssertion.replaceChildren('~header', () => ['replace']);
 		h.expect(childAssertion);
 	});
 
 	it('can set a child with prepend', () => {
 		const h = harness(() => w(MyWidget, { prependChild: true }));
 		const childAssertion = baseAssertion.prepend('~header', ['prepend']);
+		assert.isTrue(consoleStub.calledOnce);
+		assert.strictEqual(
+			consoleStub.firstCall.args[0],
+			'The array API (`children: DNode[]`) has been deprecated. Working with children should use a factory to avoid issues with mutation.'
+		);
+		h.expect(childAssertion);
+	});
+
+	it('can set a child with prepend with a factory function', () => {
+		const h = harness(() => w(MyWidget, { prependChild: true }));
+		const childAssertion = baseAssertion.prepend('~header', () => ['prepend']);
 		h.expect(childAssertion);
 	});
 
 	it('can set a child with append', () => {
 		const h = harness(() => w(MyWidget, { appendChild: true }));
 		const childAssertion = baseAssertion.append('~header', ['append']);
+		assert.isTrue(consoleStub.calledOnce);
+		assert.strictEqual(
+			consoleStub.firstCall.args[0],
+			'The array API (`children: DNode[]`) has been deprecated. Working with children should use a factory to avoid issues with mutation.'
+		);
+		h.expect(childAssertion);
+	});
+
+	it('can set a child with append with a factory function', () => {
+		const h = harness(() => w(MyWidget, { appendChild: true }));
+		const childAssertion = baseAssertion.append('~header', () => ['append']);
 		h.expect(childAssertion);
 	});
 
 	it('can set children after with insert', () => {
 		const h = harness(() => w(MyWidget, { after: true }));
 		const childAssertion = baseAssertion.insertAfter('ul', [v('span', ['after'])]);
+		assert.isTrue(consoleStub.calledOnce);
+		assert.strictEqual(
+			consoleStub.firstCall.args[0],
+			'The array API (`children: DNode[]`) has been deprecated. Working with children should use a factory to avoid issues with mutation.'
+		);
+		h.expect(childAssertion);
+	});
+
+	it('can set children after with insert with a factory function', () => {
+		const h = harness(() => w(MyWidget, { after: true }));
+		const childAssertion = baseAssertion.insertAfter('ul', () => [v('span', ['after'])]);
 		h.expect(childAssertion);
 	});
 
 	it('can set children before with insert', () => {
 		const h = harness(() => w(MyWidget, { before: true }));
 		const childAssertion = baseAssertion.insertBefore('ul', [v('span', ['before'])]);
+		assert.isTrue(consoleStub.calledOnce);
+		assert.strictEqual(
+			consoleStub.firstCall.args[0],
+			'The array API (`children: DNode[]`) has been deprecated. Working with children should use a factory to avoid issues with mutation.'
+		);
+		h.expect(childAssertion);
+	});
+
+	it('can set children before with insert with a factory function', () => {
+		const h = harness(() => w(MyWidget, { before: true }));
+		const childAssertion = baseAssertion.insertBefore('ul', () => [v('span', ['before'])]);
 		h.expect(childAssertion);
 	});