I have actually done both of these beaches with my kids (12 and 9). I will give you my thoughts on both
Playa Mia-
we booked this one through RCCL. The kids enjoyed the inflatables/ trampolines. The beach drops off into the water quickly, so not much shallow area, if that matters. There is a water slide, that the kids did a couple of times, but nothing to write home about. We had to pay for food separately and ordered a sampler that had fajitas, quesadillas, beans and rice, and it was fantastic.
Chankanaab-
Booked this one through shore excursioneer. We had no problem getting a taxi from the port to the resort. The sealion show, was okay, but I only understood half of what the announcer said. The package we bought had snorkeling and lunch included. Most of the beach is stair access, except for the lagoon area, which got really busy later in the day. The food that came with our package was pork fajitas, and was awesome. The snorkeling was fun, they have statues sunk that you can checkout.
Both are decent choices