I agree with Jim. Either resort would suit your needs, but Playa has a fantastic kids pool and sport pool set aside in their own area as well as a whole bunch of kid-related activities ( Marbella also has organized kids activities ). While I don't consider the difference between the resorts in terms of kids to be huge, I think that you might find Playa to work slightly better for the kids. My only caveat is that if you folks are a "beach family" who absolutely loves swimming in the sea, then Marbella might be the better choice as their beach is more swimmable than Playa's.
I have some Marbella Photos and some Playa Photos which should give you a good idea about how fantastic both resorts are while also showing their differences.
Enjoy your Spain journey, it's a wonderful destination.
Barry
Thanks very much Barry for the great photos and information. Just one other thing (from a borderline alcoholic!) what are the alcohol prices like (especially cocktails) and do either of the resorts have a swim-up bar