Birthday Party Dilemma

Update: thank you for all the responses, I really do appreciate all of them. I’m going to combine a few, use some of your brilliant wording and invite 5 kids and 5 parents, with clarity that extra people would cost extra. Since I have almost 2 months, I should have time to ask more kids if any in the original 5 can’t/don’t want to go. Thanks again!

Original: I find I need outside opinions because I’m torn on what to do here. My sons’ birthdays are 2 weeks apart, so this year my older is turning 5 and the younger is turning 1. I’m booking a party at a local indoor water park. It’s not crazy expensive considering that a day pass is normally $60, and the party package is for birthday boys, parents, and 10 guests (including pizza, cake, and goodie bags) for $575. For additional guests over the first 10, it’s $40 a person.

Here’s the problem: my son’s preschool class is 19 children (including him). I’d be able to pay the cost of the extra 8 kids, but then I would need to be clear on the invite that a parent would need to purchase a ticket. Or I could just pick 5 kids and tell them I’ll pay for those 5 plus one parent?

What it comes down to is my sister found herself owing a stupid amount of money for her son’s party at a trampoline park when parents showed up with the whole family plus extra siblings and she had to pay for each individual person. Quick math says I would end up owing at least $2k extra if I had to pay for two parents and siblings for this class. And no, he doesn’t have any friends outside of school.

I’d rather not change plans since this is a special birthday for both my kids, I just don’t know the right way to go about these invites. I’m hoping I’m missing something, but it seems like I either limit the kids or ask the parents to pay for extra tickets. Considering they get the park from open to close at a discount, it’s not an awful price to pay… but I could be wrong. Any advice appreciated.