Vampires are Real

Yesterday after going to Target to pick up a few things, I decided to take Zander to McDonalds to play and eat. It's been cold and rainy here for two days, so I wanted to take him somewhere that he could play for a little while. We go every now and then and he loves to climb to the top of the slide and drive in the car. I ate my hamburger and kept an eye on him while he was climbing. Then I put him in the highchair and he ate his chicken nuggets. He wanted to play some more, so I took him back over to the play area and watched him play. He came up to me crying and I didn't know what had happened. He was leaning over the slide, so I thought someone had slide down on him, but I didn't see it. I walked with him back over to the stairs and there were to many kids trying to climb up, so we went over to the toddler area. There was another boy there, but he was playing on his own. Zander went to twist the hour glass thing with balls and water in them. The other boy over there saw him, walked up to him, grabbed him by his shoulders and just chomped down on Zander. I was PISSED!!! I screamed at the boy and all the mom's came running. His mom took him and I said, your son just bit my little boy. She picked him up and said "You can't bite people, you made that little boy cry. You need to tell him you're sorry." The boy says he's sorry and he's mom asked if Zander was ok. I looked at him and said yes. Then they went and sat down. I couldn't believe that she didn't do anything else. He didn't get time out, she didn't make him leave, she didn't do anything! First of all I couldn't believe he bit Zander right in front of me and then he's mother doesn't really do anything about it. She told me he doesn't "use his words" very good and they were trying to make him "use his words." She also said he bites his brother but he has never bit anyone else. I just couldn't believe that she did nothing, really, and that she wasn't really keeping an eye on her child. Later when I got home, I looked at his back. There was a mark in the middle of his back where I suspect the boy bit him when he came running to me when he was leaning over the slide. The mark on his shoulder was pretty ugly and it just made me more angry. What was I to do though? I really had a hard time not smacking that kid in the mouth. He had to be about 4 or 5, so he knew better. What would you have done??? Here's a picture of his arm: