Per parties at venues:
1) You pay for your child and the kids, that are invited.
NOT the parents.
Not the other adults who are tagging along.
The parents will pay for their own admission.
And your relatives as well.
2) Will you also provide a lunch or meal? So then in this scenario, you pay for your child and the kids who are invited. Per what is offered at the venue. Or you have to bring in your own food. And you have to check to see if this is allowed at the venue.
You state, that parents or others, will pay for their own meal.
3) IN the invitation, state that: "party will be at water park. We need a head count ahead of time, if your child will be attending. The child will be paid for. Any parent that is attending will need to pay for their own admission.
Also state in the invitation: "Johnny and 1 parent is invited to Eric's 5th Birthday party."
ALL of my friends, do this in the invitation. Therefore, you "control" how many attends along with the invited child to the party.
4) Send the invitations, 1 month or 3 weeks, AHEAD of the birthday party date. SO THAT... the families can figure out their schedule and/or any babysitting they may need for their other kids. If need be. Or make their Husband stay home to watch their other kids.
If you send the invitations out at the last minute, you reduce the chances of people attending.
5) Now, it is not only the water park admission you will need to pay for... it is also for food/cake/goody bags if you have that, too. And transporting it to the venue or not. Or unless the venue 'caters' food there too. And what is that cost?
6) Water parks, are pricey. In my city, our water park costs more than that. And yes, some families may not... want to pay for going. Or they may not be able to afford it.
So you really have to think about this and if you yourself, can afford this type of party.
7) Per our kids, we always ask them what kind of party they would like. BUT... we always also "discuss" with them... about budget/what we can afford or not/and about how many kids they can invite. AND depending on this.... our kids and us, decide about the party. And they are always very amendable to it and understand. And they Always, have a party that they love and they enjoyed it. Even if, we make a party that is not as lavish as they originally talked about.
8) You also have to state on the invitation, the time frame of the party.
When it starts and when it ends.
And you will have to clean up afterwards. And then lug everything back home.