The dog get my peanut!
This one requires some context to be really funny.
My boy is 3 and is still working on speaking well. We have him in speech therapy in fact (going great, thank you for asking) but like many kids his age he substitutes some words for similar sounding words.
We're also in the middle of potty training, which is also going great. Recently, I asked Bo to sit up on the potty and go potty and he climbs up (he likes using the big potty, not the one we bought special for him). So, he gets up there and is about to go and I say to him, "Bo, you better hold down your willy there."
Without skipping a beat he says, "Hey! It not willy, it's a peanut."
So, when he is done taking a bath, and is naked running around the house, and he sees the dog, he is liable to put his hands over his, erm, peanut, and say the above statement. Which, honestly, is a pretty logical fear because that damn beagle will chew on anything.