Mama Bear rears her ugly head

I can not handle when other kids hurt mine, I know this is something I need to get over because obviously having boys they are going to get in fights and have kids bug them, but I am still not ok with it. We went to Stefanie's today and then decided to go to McDonald's with the kids. When we walked in of course the kids had to run into the play area but I figured it was fine

since there was only 1 door you can go in and out of and also because Rex was with Derek and Daeson. So Stef and I order our food and as I walk into the play area I hear my son SCREAMING at the top of the big toy. Of course I can't see him because he is those tube things. So I put my food down and climb up into the toy, not an easy task for an adult. I get up there and find Rex by himself just screaming with snot running down his face. He is crying too hard to tell me what is going on so I slide down the slide with him. Well at the bottom of the slide is this lady with her kid who is probably 4. She asks me what happened and I said I don't know and she looks at her kid and asks him if he hurt Rex. The kid gets this guilty look on his face and says no. So then this other lady comes over and says I saw him up there kicking him. Lovely! Well the mom was ticked at her kid and storms off with him but then 10 minutes later she lets him keep playing. In the meantime I am fumming and I just want to go over and yell at the little bully but instead I keep giving them dirty looks. Poor Rex to have some big bully beating him up for no reason. I know Rex didn't do anything because he is such a sweetie he never does anything unless he gets hit first. I understand a kid hitting him now and then especially when they are around his age or younger but this kid was old enough to know better and I am NOT ok with kicking my child. AGHHHH! I am fumming just thinking about it. How am I going to handle when they are in school? I better toughen my skin I guess. :{