i sssoooo know what you mean
It's very frustrating, we went to a party today infact, where I ran into the dad at the store a few hours before the party, he said they only got 5 RSVP's out of nearly 40 kids, 1 was the birthday girl and 2 were my own kids, about 12 kids showed up.
Personally, what I do, is put a lil RSVP card inside the invite. It's super simple with preschool ages. Parents get the invite from the ped-board outside the class, open card, check yes or no, put the return card under my child name on the pegboard, done.
It was very cute for DS5's party, it was pirate theme, so the RSVP cards said something like :
_____ Yeah's me climbing aboard with me's crew of #______ mates
_____ Na's me's walkin' the plank
They just checked yes or no and how many people were coming.