Toddler Bullies

I just had my first experience of watching my son beat up by another kid and not able to do much about it. It was awful.
The scene:
We are at McDs playplace. There were lots of kids. Lots of boys. Rowdy to say the least. My boys were doing great. The usual, they pick a friend and follow them around. Running, climbing, go down the slides. John usually picks the only boy who is older than the rest to befriend.
After a long time of playing I could hear Johns low, moaning cry. (He doesn't scream like Jake does.) I think the moan-cry is partly because he's embarassed for other kids to see him cry. I heard his cry and when I finally found him he told me his friend had hurt him. I knew the boy (later I find out his name is Maxwell) hurt John because John is tough and usually only cries when necessary (unlike Jake.) I hugged John and told him to find a new friend to play with because I didn't know Maxwell's mommy. Then John said, "no." and gave me a look like 'why don't you do something?' So I turned to Maxwell and said, "did you hurt John?" as sweet as possible. I do not like disciplining kids who are not mine, especially strangers. What if his crazy white trash mom was watching me? And then she would plot to meet me in the parking lot and jump me. Anyway, Maxwell said he didn't hurt John. I knew it wasn't the truth but what could I do? So I just said to both boys, "let's be nice friends. no hitting. no hurting each other." again, as sweet as tone as possible. Ten minutes later....
There are about 5 boys bouncing in this little play-cove. They are all rough and tumble. It's hard to stop it because the boys love this stuff. I watched carefully. Jake got hurt twice, I pulled him out while he screamed. As if that wasn't sad enough, I see John get stomped on by Maxwell. Hard. Then body slammed. Then his hands jumped on. All on purpose. John was crying out in pain and screaming. I was banging on the window yelling--really yelling--at Maxwell to stop. John was bawling--silently at this point. That's when I know it hurts--no sound comes out of a terrified purple scream face. I'm telling John over and over to get out of there. Finally I start to climb in to pull him out. Two other kids were on there way out and therefore I couldn't get in. At this point Maxwell has stopped stomping on him. He was staring at John who was bawling in pain and screaming, tears flowing. So what does little devil boy do? He kicks John as hard as he can in his ribs, twice. I swear I felt it in my own ribs.
I screamed at him to stop. The whole playplace full of parents staring right at me. I get John out and my adreniline is pumping. His mom rushes over and says, "what's going on? who did what?" etc. I said that her son was kicking mine, not even doing his crime any justice. So she says to her son, "did you kick this boy?" Of course Maxwell says, "no!"
I couldn't help it. I had to tattle. I said, "yes he did. I saw him. Several times."
After a tear-filled packing up and putting coats on, right before we left Maxwell and his mom came over and she forced him to say a reluctant "sorry." I forced John to murmer a reluctant "I forgive you."
I'm not judging Maxwell's mom. I totally realize the situation could be reversed. But in the end, I was just so sad to watch my son stomped on and then kicked in the ribs by a boy that he wanted to follow and have fun with. Poor Johnny.