I agree that these probably would be bringing in more money from rec., open gym, and fitness class members than team kids, so it could be way to bring in money. Experienced gym parents wouldn't go for these things, but rec. parents eat this kind of stuff up. And I also agree that it doesn't need to be any special kind of label, just a store bought bottle would work and up your revenue. And bulk snacks to have on/behind the counter for class kids and younger siblings could be a good idea. It takes away the mechanical issues of a vending machine but still brings in money.
Reusable bottles are a great idea, especially the good quality bottles in fun colors. For reusable bottles, or other items with the gym name/logo, parents eat that up. The cheer gym I'm at one night a week sells shirts, shorts, socks, hair bows, bracelets, tons of silly little things with the gym name on it in the gym colors and they sell like crazy. Almost all of the class kids have at least one club t-shirt or tank top. You can easily mark these things up 50% (more if you find a good vendor).