Two very different places. Playa Mia is a beach club with water toys, a kiddie splash area, etc. Some cruise lines have excursions there, so it will likely be crowded.
Chank is a marine park that also has a beach - admission is about $20 each, but they offer a beach, separate snorkel area, wildlife shows, nature trails and the opportunity to swim with the dolphins (big extra fee!).
What do your 11 & 12 year old like to do? Good snorkeling for less money at Money Bar (no beach) and other spots. Better beach clubs (Paradise Beach offers better facilities, better food, and a HUGE pool) for those without toddlers.