Uggg, I had this happen to me once almost 2 years ago exactly. (DD's b-day is next week.) We went to a bounce house, it was closed to the public and only open for parties. On this particular night we were the only party. No one said anything bout bringing siblings but 5 or 6 showed up that night. Since it wasn't open to the public they were going to get charged the party goers rate. Out of all of them only 1 person offered to pay for her child and she was a friend of mine, of all people her child is the one I wouldn't have minded paying for. I was so caught off guard I didn't know what to do. It was bothering me all night through the party, my DH said let it go, we will pay for it, but that was a lot of extra money, I think like $80. Last year I was so turned off I didn't give her a party, and I felt awful about it but I just had a bad taste in my mouth after the other party.
We are having a big party for her next weekend, I let her invite the class and a few other friends from scouts and dance. We are having it at Chuck E Cheese and not doing a party, just using a bunch of coupons. I plan on having extra coupons there for any families who show up. I can say "Oh you are staying? Great. Do you need coupons?" I won't do a private, I am stuck paying place again.
ETA- This is the first time we are inviting the whole class out of 3 kids, I guess I let her becuase I felt bad she didn't have a party last year. I don't suspect many of them will come. This has been "the thing" this year though to invite everyone, never ocoured to me to doo it before. Thing is, the last party my DD went to was for a little boy. We were free so she went, she was the ONLY one to go. He had a neighbor and a couple cousins there, but out of the whole class she was the only one to go and the mom said not one person replied other then us. I will plan for everyon but won't be buying food till people get there. If 25 come great, if 2 come great, won't matter.