K.P.
I would call and RSVP and ask! My guess is that they are doing pizza, cake and presents for the kiddos, but you could certainly let the mom know that you are a little nervous about leaving him. She will likely give you the "agenda" and invite you to stay.
If she does invite you to stay... don't bring any siblings. If she doesn't... up to you. It's probably not all that different from leaving him at the preschool, thhough- when you get there if you get an "unsupervised" vibe... offer to help serve food and stay.