If possible, let your child have a favorite special "backyard cup" she uses for keeping hydrated in the heat. One with a cap and straw is ideal, or for younger tots, a sippy cup is good. Fill it with ice and water, and let him be "in charge of remembering to drink often" like a big kid! Having a fresh fruit snack in the fort or at the playset picnic table is fun too.
Tip- if the slide is very hot, hose it down before use.