S.N.
I'm going through this right now.. but we have a list of parents/emails/phone #'s that I will be calling from on Mon. (party is Fri.)
One thing that we'll be doing b/c we have to pay for a min. even if they don't come is inviting the children's younger siblings .... just to not have to pay for 25 and only have 10 show.
If I didn't have the list I would talk to the teacher and ask if I could put a reminder in the kids backpacks (if everyone is invited I believe they will let you do that) AND I would get there early to pick up/drop off and start mentioning it to the other parents.
Start engaging them in conversation and saying something like "I can't remember, are you guys able to make it to the party on Fri.?" I recently did this and 2 of the mothers said they were coming... but still haven't RSVP'd. AHHH. Good luck! SMK