Is a pool party or bday at a water park odd on Sept 1st?

C

So our daughter's birthday is September 10th, and the end of the amusement park/water parks season in our area is September 1st. Her "friends" are all older 4, 5, 8, and 12. I was thinking a water park so that our daughter can go with us on the lazy river, while the other kids could do what they want and still have fun at a baby's party. Any other ideas are welcomed!

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