Hi J.,
I never had a problem throwing a party together with my ex, but he never wanted to to. For whatever reason, he refused to tell me. BUT, I always invited him...and his girlfriend. But anyhow, that doesn't answer your question...I always invited the kids from school to my party. He usully threw a party, but just his family and friends were invited...I am more involved in the school life of our daughter. I know the parents better, the kids come to my house for play dates, I'm the room parent...etc etc. It was never an issue because he had plenty of friends with kids and plenty of family to make a party for her.
Now, with my step daughter...(that lives with me)....again I am more involved in the school than her Bio-Mother, I am the room parent, kids come to my house for play dates, etc etc. Again, we invited the kids from school to our party..but last year, all of a sudden, bio mom got competative and wanted to throw the same kind of party we were and invite some kids from school...not that she didn't have a right to throw a party for our daughter, but that's another story...lol...so to benefit my baby, I told her Mom that was fine, but we had our daughter make two lists of people, one for our party and one for her mothers. We mailed the invites, so people didn't get confused or feelings hurt..etc etc. Worked out fine, she had a great time and both parties!
Good luck, and with time it usually gets easier dealing with this stuff and an ex...
D.... :)