Let's Get Together: DIY KOOB (The Best Lawn Game. Ever.) - It works for 2-12 players (what a range!), ages 5 and up (kids and adults love this) on any outdoor surface (grass, gravel, dirt, concrete, etc).
would be fun to do as a relay. The first kid starts with a full cup of water, then pours it over his head and the second kid catches it and repeats and they keep doing this to see who can fill up a jar the fastest
Ice painting is a fun way to cool off during the hot summer months, and this website even has a substitution for children under the age of two who will want to put the ice paint in their mouths: kool-aid ice!