H.H.
Does your child know all the kids in both 1st grade classes? Maybe you could invite the class she is in and ones she knows from the other class. From my experience not everyone will make it but with it being a pool party you will probably get a good response and a lot of parents will probably want to be there especially if their child isn't a real good swimmer. You can put in the invitation that you are paying for the child and they are welcome to come and swim but need to pay for their own and any additional family members. As for the limited 25 people area. I don't know what to tell you.
I reserved a pool and paid a flat fee. not per person fee and could have up to 120 people there as that is how many the pool could have so all their families were invited. All of the families didn't come but there were several that did especially if they knew their child wasn't a good swimmer.
You will probably not have too much time for games but if you wanted to do one. Maybe you could have a relay with grass skirts and lay necklaces. They run down to the other end of the room put on the grass skirt and lay and run back to their team and take it off. The next person puts it on and runs down to the other side and takes it off and runs back to get the next person and keep doing this until the first team gets all of theirs done.
Is there life guards? With kids that young you want to make sure you have enough adult supervision to make sure they are safe. Most kids that age do not know how to swim on their own and some will get brave and jump off in the deep end not realizing it is over their heads.