Ideas
Saves are not even needed with this one: just go on the gallery, and download families with children. Some of them will have this issue (tested with families in my gallery - Down-In-Simsland, or my brother - simsontherope).
So it's purely an hypothesis at this point, but seeing it affects all my children makes me wonder: I always create children in cas as teens, modify their features for them to look good when they will age up, then turn them into children (without saving the family or going in-game in between). Maybe that's why they retain some body features they are not supposed to show as children as body sliders should be unavailable / not displayed.
I never do this (create children as teens and age them down) and I have some children who look skinny and sort of stretched a bit. A lot of them are made over Maxis premade families that have been in my Library for years.
The toddlers I checked were all adopted or born in-game. Toddlers have definitely lost the chubby aspect to their faces and their eyes are smaller. My toddler's bodies were all okay though.