What happened to my 24 hr chick?

While back Buff got broody. Sat for a week. I was all excited. Went out a week later when I caught her eating and as you can guess. No eggs. But she went right back in to sitting. So I got my other buff egg that day and put it under her. Next day same thing. Well the first egg I found broken at about 2 weeks. But yesterday at 21 days baby chick. It was healthy and under mom. Well tonight I couldn't hear the chirping and hubby and I checked and it was dead under mom. Important note. We have a pair of bantams. And for some reason the bantam started laying eggs under buff last week. I did not know this and was on vacation. Yesterday when I checked for the chick mom had bantam eggs under her. She is still sitting. I am wondering if the sitting on the eggs is different than the way she sits on chicks (I would assume so) and if that smothered the chick. I know they keep the chicks under them but would assume they would have to let more air in for the chick than an egg. I am checking for new eggs so that no more are laid under her. We are very upset. And have to give my 8 year old an answer in the morning. And would like to have a real answer. Welcome to the world of chickens.