I would have thought that if you receive an invitation for a birthday party that has an RSVP date and contact numbers on it that it would be polite to let them know if you will be attending or not.
For JJ’s fifth birthday party I’ve invited quite a few kids and not all of their parents have let me know whether they are coming or not. If I didn’t have to worry about catering and game prizes and stuff I wouldn’t care, but I do and I object to having to chase people up. In fact, hopefully not to my detriment, I’m not chasing people up.
I just had this same situation! You have planning to do and need to cary on with some idea of your numbers. UNfortunatly, you may have some show up that didn’t RSVP. The kids are only 5 so the responsibility is on their parents but it would be a shame to have them singles out or not have the same treats as the others.