The problems I found was clearing their ears.
It's OK if you descend slowly, say -300fpm, but if you are airways and ATC give you a last-minute descent which needs -1000fpm or more then the kid may need to consciously clear his/her ears and the very small ones are not capable of understanding how one should do that (hold the nose while swallowing, etc) and it can get very painful. I once had to level off on a STAR and was about to tell ATC I could not continue the descent when the problem got sorted. That one was a bit "urgent", one might say... today I would not carry children on an airways flight unless they have that level of understanding.