S.
Unfortunately, many people do not always respond to the RSVP dates. Also, I would be very careful about filling those "vacancies-" someone's feelings could really get hurt (parent really, a child could care less) if she finds out her child was on the "B List."
Good luck -- it is a 5 year old birthday party. Have fun, even if you don't get the targeted number of children, just make sure your child has fun and feels special. It really is his day : )
Congratulations on the BIG DAY!!!
S.