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Yeah, I agree. Don't they look a little too old for the age they're supposed to be?

I hadn't thought of that (facial proportions) but yes, they appear wrong. Eyes, nose and lips should probably be larger in relation to face size, since eyes stay relatively the same size as the other features develop around them. Even the boy's body should have more stomach / intestine happening in the abdomen because younger children don't have so much of the "upside-down-triangle" thing happening in their torsos.

Regarding the toddler: granted, I don't have children and I know that hair's a prop but that's a lot of friggin (very adult looking) hair.