SO seeing his ex?

My boyfriend and I are going to a kids bday party tomorrow (along with our daughter) and his ex will be at the party as well. They dated about 2 years (his longest relationship and ended about 4 years ago) I might be over thinking things and just insecure. But would anyone else be worried about him seeing her? I'm scared that once he sees her his feelings will start coming back. Am I over reacting and should I just relax? Or would anyone else feel the same? How would you feel?