I run Time4Learning.com which has a kids playground which is really for the youngest kids (preschool up to second or third).  Our third through eighth graders, actually their parents, have often asked for a kids playground for them.  When I speak to them about it, they're really thinking of an appropriate combination of:

- casual games
- a community with forums, blogs, and other fun ways to interact

I've tried to find a community design that i could use but so far, i've never found one that met both our technical and safety requirements.  I'm getting ready to rethink it.  Points that I'm sure of:

- Parents need to be able to supervise if they want to.
- Parents can just trust us to supervise it
- There need to be two distinct areas with entirely different look and feels activities: a 4th-5th grade, and a middle school area

I'll go looking at ning and epals and some other ideas. I might also start by looking to see what kids are doing these days on blogs. What are they writing about at those ages? What do their memes sound like?