Since you are staying at the CR already - I would have to agree - the Beach Club would be a better choice for your second resort.
You'll already have done a monorail resort - so while you are there, you can concentrate on the MK, and go to the Poly for dinner!
When you move to the Beach Club - you can concentrate on MGM and Epcot, without having to deal with the bus to/from MGM. There is more nightlife in these areas for your preteen/teen, and the pool is supposed to be awesome (haven't stayed there yet - just CR, Poly, AKL, and AS Mu).
Either way, they are both wonderful resorts - so have a great trip!