I was too fatigued to watch the baby at the lake, so I let my toddler keep an eye on her
That sounds dangerous. Do you really trust your toddler to watch over your infant daughter?
I trust my toddler all the time.
It's wrong and inappropriate to delegate your parenting responsibilities to a toddler since he or she does not have the ability and life experience necessary to make good decisions regarding the safety of your infant daughter. Your toddler may not know what to do in an emergency and that could lead to your infant daughter getting injured or even killed. Instead of letting your toddler babysit, you should find a responsible adult to do so when you are tired.
Whatever, it's not like I'll let a two year old drive a car.
Even if you won't let your toddler drive a car, don't you think that you shouldn't delegate your responsibilities as a parent to your toddler? If someone saw you leave your toddler to watch the baby at the lake, they might have reported you to the authorities. You could even lose custody of your children.