Kid Bullying

I'm interested to know how you all deal with this problem. A blog of my friend's addressed this issue and there were a lot of really helpful comments.

Let me explain. Charlie (13 mo) and I went to the library for story time yesterday. Everything went fun and dandy. Afterwards, we went to venture to the play area where they have those fun tables with the beads on wires and the sand/magnetic thing that you move the cars/whatever from the bottom with a magnet. I was sitting close by in a chair and there were about 3 other kids, including 15 mo. (I asked) twins and, I guess, a 4 or 5-year-old girl. The twins were being supervised, of course, but the 4 or 5-year-old girl was not. Charlie was having fun cruising from one table to the next and exploring all his options. The little girl was playing with the magnet thingy. Charlie watched her and as soon as he showed some interest in the magnet thingy by just looking at it, this girl started screaming/yelling/throwing a fit and then forcefully pushed Charlie down so he fell down on his bum. He got really upset and started crying big time. I think he had his feelers hurt more than anything. I ran to the rescue and helped him up. I continued to sit right by his side and he calmed down. I said something to the little girl like, "He wasn't trying to take it away from you." A couple minutes later, the "mother figure" showed up and asked if she had done anything to hurt Charlie (as the whole library could hear him crying). I didn't want to make her feel bad and replied, "He got pushed down," instead of an accusatory, "She pushed down my baby!". There was a forced, "Sorry" and the mom was gone again. Again, Charlie stepped too close too the toy she was playing with, and screamed/yelled/threw a fit and begin squeezing/strangling Charlie's neck! It happened so quickly, I couldn't believe it! Luckily, I hadn't moved from Charlie's side and was there immediately to pry her fingers off my son's neck. No mother to be seen. Now, here is my question, if you read my friend's blog you'll see lots of solutions if the mother is actually watching her child misbehave and hurt another, but what if the mother is no where to be found?? I am a "I should have said/done _________!" kind of person, after the fact and then I stew myself to bed thinking of all the things I should have done, getting madder and madder. Obviously, the safety of your child comes first. We left the library right after that. I did not say anything to the mother but I felt that I should have said that her child was acting violently towards my baby and that she needs to be more closely supervised. The more I think about it, the more I have the feeling to slug that little girl! This isn't healthy. I know that someday Charlie may be on the other side and may hurt someone else, which I will deal with as it comes. But I just feel I should have done more than just pick up and leave, with no consequence to that little bully. Alright, all you playgroup gurus, there has to be some solutions out there.