A Button is an interactive Mono Glyph. Most of the time, one would simply create a Label and use that as the Button's child Glyph but, in fact, any Glyph can be a Button's child, even another Button. But something to understand is that if a Button is within a Button, the inner Button will never receive any input Events (or characters). This is because when a Viewer's 'run()' method first starts, it walks its child tree of Glyphs recording each descendent that is interactive; each descendent that can receive Events; each descendent that can gain focus. During this Glyph tree walk, if a container Glyph (Mono, Poly or Grid) can accept focus, the tree walker does not descend into the container Glyph's children. Thus in the case of a Button within a Button, the outer Button will receive the Events when it has focus and the inner one never will (unless the outer Button decides to pass them along of course).
So, what else might make a reasonable child Glyph for a Button? One could certainly put a Label in a Box and then add the Box to the Button. Images and bitmaps are not supported by Forms so, while most GUI libraries support Button with those child types, the Forms library does not.