Why not help them write their own plays? I've participated in several twenty-four-hour play festivals in which the plays are written between midnight and six in the morning, the actors and director rehearse all day, and then the plays are performed at night. A week-long camp seems like a great opportunity to practice something like that.
If you'd really like to use existing plays then I recommend acquiring a play collection by Christopher Durang. His work is very funny and quite simple.