Public Park Sprinklers. I've been walking in the park many times when suddenly caught unaware when the sprinklers go on. These sprinklers are meant to keep the public's lawn green, but there's nothing stopping you from taking advantage and letting your kids soak themselves to the core. Either figure out yourself the sprinkler schedule, ask park workers, or even call up city hall to find out when the water will be spraying. Come to the park prepared in lightweight, easily drying clothes, and let the fun begin!
Watering your Garden. Not enough rain, so you're watering your garden or your lawn? Use a sprinkler and let the water accomplish 2 purposes simultaneously- quenching the plants' thirst, and cooling down your kids.
Bath Outside. I bathe my son in a wash basin on a regular basis. Simply moving that basin outside and using cool water for the bath instead of hot is enough to turn a regular bath into a private wading pool. Even if you don't regularly use a wash basin for baths, making your children's bath occur outside instead of indoors is a decent way to keep cool without wasting water for extraneous purposes. Of course, this will only work for children young enough to go outside less than fully dressed.
Playing in the Rain. While it doesn't rain locally in the summer, if you live in an area graced by summer showers, play in nature's sprinklers for a cheap, refreshing activity.
Do you and your kids play with water in the summer? Do you use water specifically for play, or do you try to find ways to multitask so you're not wasting water?