K.B.
I would be upfront about what you are doing that day and say I will be bringing the lunch and snacks for everyone but the admission fee for the pool is $7, if this is a child that comes a lot with you to the pool maybe let the parent know they can buy a pass for the child for such and such amount which would save them money in the end (guessing).
To have time to myself I would send the money, maybe even a little extra for a specail treat for everyone, along in a heart beat!