I don't really have a lot of Sims kids - and so far they are normal size - no really chubby ones or too skinny ones - so I really have no idea.
I don't play big families though - so I keep an eye on my sims families when I am playing them and have them all pretty active and skilling a lot. I have them do the physical skills instead of just reading them. It might make a difference. They are never idle except to sleep.
I do not add a new baby to a family until the last child is ready to turn teen and has all the skills they can earn. So I watch my sims closely - perhaps that helps to keep them more normal size.