Party Invitation Wording Help

(Texas)

I need some advice! We are holding a birthday party at a pool, and of course there is an admission fee to get into the pool. I was wondering what the etiquette was for paying that, and how to word it. I assumed it was expected to cover the admission fee for all invited kids (in some cases this will be 2-3 kids in a family), plus one parent. I want to be sure folks know the other parent is welcome, as would be siblings (especially younger) whose parents can't or would not want to leave home with the other parent or sitter, but obviously that is really going to ratchet up the cost if we pay for every single person, especially since we will be providing food and drinks. Is it expected that we pay the admission for all who attend? Or is it ok to ask additional attendees, while welcome, to pay their own admission? If so, would it be acceptable to say something like "Our party package includes the admission fee for all invitees and one parent per family. Additional parent/siblings are welcome to join us, and the park admission fee would be $6 per adult/$5 per child."? If not, what are some other solutions? This is my first party where there is an individual admission fee, so any suggestions on etiquette would be appreciated. I don't want to offend anyone! Thanks!!!