Dealing with bullies

Today I took my 3 year old son to an indoor playground. Its at a place we've never been to out of town so we didn't know any of the kids. My 3 year old was playing nicely (he's the type of kid that will want to be friends with kid he meets and isn't shy at all). This older kid, about 7, came up and asked him to play. My son was so happy so they went to the top of the structure (mesh, so I saw everything) and out of nowhere the older child pushed my boy down and started kicking him. I climbed up that structure so fast and grabbed my son. He was screaming crying. I stayed with him after this and didn't let him play on his own. The boy ran off obviously and no parent in sight also. I was very upset but my husband told me to stay calm and not let my anger show but I was so angry, to a point I just wanted to break down and cry. After this, my husband stayed with our son and I sat with our infant stroller at a booth by the structure. As I'm typing this, any time the boy passes by me, he gives me the finger and laughs. How do you deal with this? I'm angry that someone hurt my toddler but I also don't want to be so emotional about it. I'm just ignoring the kid but I'm very upset at the moment and idk if I'm being childish by being so emotional.