Not sure what a cooling towel is. I've used those bandana/headband things that have absorbent material inside and can be soaked in water, which is then retained. They are good around the forehead and the neck. Is that the same thing as a cooling towel? Look in the beach and camping sections of stores.
I don't love Gatorade because of the sugar and dyes. I use an isotonic sports drink with fructose (not high fructose corn syrup) and beta carotene. There's a small amount of sodium to replace what's lost, but not so much that the kids keep getting thirstier.
No caffeine based sodas at all, either before of after exercise. If kids really want a treat, flavored seltzer gives them the bubbles but mostly water and a little flavor, no calories or sugar. It can give them a belly ache if they are burping while trying to run around, but for afterwards, it's fine.
A large container of water for pouring over the head is important. Frequent breaks, hosing down (a lot of our kids wear the breathable fabric shirts so their clothes don't stay drenched or dripping), and shade (those pop-up tents are great on the sidelines).
Watery fruits like watermelon and oranges (serve in wedges) or clementines/mandarins (easy peel). Sports teams and college track teams use the oranges a lot - kids can just suck on the open fruit if they're cut in quarters, and they also get the fiber (which they don't get in juice).