We call the first trick "Ice Mamma." Fill a 2 liter bottle with water, and freeze it solid. Put it in with the chickies on a hot summer day, and they will cuddle it, climb on it, rub themselves aginst it, and treat it like a beloved mamma! And they'll cool themselves off in the process! Keep two or three of these going in the freezer all the time, so you can switch them out as they thaw.
Trick #2: Frozen fruits and veggies. We freeze watermelons, cherry tomatoes, cucumbers - whatever you have an over abundance of in your garden, freeze it for a few hours and put it in with the chickies. They love it! And, of course, it cools them down.
Some people say we spoil our chickens, but these are cheap and easy ways to keep young chickens cool and healthy in a heatwave. And why not want that?