There are plenty of great play scripts written just for kids by kids on this website. Through the DramAntics program and an earlier incarnation of my playwriting program I ran with the Girl Guides - I have loads of scripts just waiting for another stage. So if you have a youth theatre, a youth group, a class, or a bunch of kids in a basement looking for play to put on - check out the scripts. They're free, easy to stage, and easy to memorize. In fact, the majority of the plays were staged in just one week.
If DramAntics can do it, you can do it too.