I'm just home from a local under-5's play group and shit went down 
My 2yo DD was playing on a little ride on thing, my friend and her 10mo were just to the side of her, and I was just across the room with my 8mo, I could see my DD clearly.
Everyone is playing, and a little boy, must have been around 3/3 and a half, grabbed my daughter by the neck and pulled her off the ride-on toy and pushed her to the ground! My friend picked my DD up just before she cried and told the little boy that that's really not a nice thing to do, but before I could get there or she could stop him, the boy picked up the ride-on and swung it in a rage straight into her 10mo's head 
The baby had a cut on his head that was bleeding quite a bit and a big bump on his head! My DD had finger marks and scratches on her neck. The baby was cleaned up and logged in the accident book, but when we returned to the main room, the little boy and his mother were nowhere to be seen.
I don't know if they were asked to leave by staff or if she chose to leave, but there was no apology at all. If that was my DD who had done that to another child I would be apologising forever to the mother!
AIBU to think she should have apologised and maybe checked that the baby was ok?!