I listened to a podcast that interviewed Barbara Tversky, a psychology researcher specializing in spatial reasoning.
She mentioned at one point an explanation of this is that your sensory/motor neurons are not evenly mapped across the surface of your body. That is, your brain devotes far more sensory neurons to your fingers than say, your back. The "resolution" is higher in certain important areas, and this might explain why kids' drawings almost universally have giant hands with fingers, and giant heads, and de-emphasize things like the torso.
That might be overthinking it. Hands are (infamously) tricky to draw. It's a natural mistake when drawing to make the things you spend most of your time drawing the largest. It's embarrassingly easy to spend a lot of time drawing someone's hand or face and zoom out to realize you've gotten it completely out of proportion. To me that's quite enough to explain why hands are often drawn out of proportion.
If you're insulting my writing, we'd both be better off if you were more explicit (I have to admit that second sentence is a bit of a mess). Otherwise I have no idea what you're on about.
I don't think this has anything to do with childhood. It's just a matter of illustrating the shape. Fundamentally, most people don't so much draw things as crudely diagram them.
I've always wondered what the developmental psych is behind that. Maybe their perspective of adults?