I recently saw an article about a child who no showed to a birthday party and the parents sent the other child's parents an invoice for the amount they paid for his attendance.
Even though it seems ridiculous to send a bill for not showing up at a birthday party and unfair to drag the children into it, I actually have to sympathize with the parents who sent out the bill. It is so rude when people say they're going to show up for a party, you pay for them, and then they don't show. I probably spent an extra $500 at my wedding for people who didn't show up. And didn't even apologize for it.
For a birthday party, the amount of money is smaller, but really, it's the principle of the thing. You expect a certain number of people, you make an amount of food for that number of people, you pay the host, you make goodie bags. If you're not going to show up for the party, you better have a really damn good excuse. Not just "there was something else I felt like I'd rather do instead."
This is why, at an upcoming birthday party I'm planning, I am underestimating the amount of kids that will come. I'm sure they can make room for more kids at the last minute, but I'm not paying for the inevitable kids who don't show up.