Not surprised with this situation at all. The few times I've been to the mall play area, I've seen crazy stuff.
If any child is being very aggressive towards others (except maybe siblings and parents are there), I would say something like, "Hey there. We need to play in a more gentle way. There are too many littles in here. We don't want to hurt them, do we?" By using "we" it takes the sting out of "YOU" and makes the message less confrontational.
I would have also wanted to watch to see how it played out and be there to explain to the absentee parent what their child actually did do. I think the boy needed reprimanded, but maybe not as harshly has the other woman did it. But, who knows? Maybe that kiddo will remember that very well.
If hubby wanted to pull us all out of there, I would have insisted stopping at a mall kiosk and telling them that some craziness was going down at the play area and they may want to send security over there. I would even leave my cell number if I felt that my eye witness report would be helpful.
Tough situation for sure. Now, if I knew for sure that the parent was not in the area and no reasonable caregiver was present, I would contact mall security and let them know that a young child was left unsupervised (and was being very aggressive towards other kiddos). That is not a safe situation for that child. It was about a year ago that a 2 year old was abandoned at a McDonalds and no one noticed for most of the day. How sick is that? I'm a watcher and like to know that all kids are accounted for with a caregiver (whether they suck at it or not). I do believe it takes a village....