We went to Playa Mia, on our own. We booked on their website and took a taxi there. Way cheaper than booking through
DCL, and you have the advantage of coming and going when you want. There were times when it was quite busy there with all the cruise ship tours, and other times where it was nice and quiet.
The beach is okay. Nice but not spectacular. my kids, who were 2 and 5, enjoyed it.
There were various activities going on, like water balloon toss, becah volleyball etc, that your kids might like.
The pool and waterpark were definitely a plus. The water was pretty cold though. It was a cool, windy day when we were there (probably 20-21 degrees C), and because the water was cold, we didn't spend long in the waterpark or pools. From memory, there were about 2-3 waterslides that would be appropriate for your kids; the rest of the waterpark is more geared towards younger kids. But there is a pool.
The buffet food included is just horrible, but the food you could order and pay for from the restaurant looked much better. Were I to go again, I would still book directly with Playa Mia, but I wouldn't choose the all-inclusive package. I would just pay for the food myself.