tully - From what I understand, the wildlife would be redundant at MA compared to Osa, but if the OP is looking for watersports/swimming for their kids, wouldn't MA be a better choice? I just keep hearing about non-swimmable beaches with riptides in Osa.

hip - true, true, I guess it would depend on how much beach time you want; if that's a primary focus I would say MA. The beach where the Black Turtle is is near the Iguana Lodge, and it is swimmable in that area though not calm, a nice long stretch of beach.