We went a couple years ago (although we were in an all-inclusive a bit south - in Riveria Maya) and my daughter who as 8 at the time.
I would say it depends on how long you are there. We were there 7 days and I was fine to hang in the pool/ocean the entire time. There is SO much to do just at most of the resorts and/or shopping. I'm a scuba diver and I did dive a couple of the days, though.
However, we took 1 day and went to Xcaret. http://xcaret.com/
She had a B.L.A.S.T. we snorkeled in this amazing river/caves with fish and treasure. The animals are fantastic and we saw a terrific show with great costumes, fire and music!!!!
It's kind of pricey - but definitely worth every penny. However, you have to be careful about what's included. They have SO MUCH that is free, but then they offer packages of stuff that is almost duplicate that you have to pay extra for - so be careful.
We will be going there again with our family next year, and I am considering just staying at their all-inclusive resort that is connected (so your all-inclusive fee includes admission to the park).
I wouldn't suggest the Maya Ruins. It's a TON of walking and it's VERY hot and dusty. My daughter was really interested in the people and the culture and would have been bored-stiff (the performance at Xcaret gave GREAT information about the history and cutlure of the Mayan people and she LOVED that). My mom, bro & sister-in law went one of the days and said most of it is now roped off and you can't even get to the good spots (they had gone previously a few years before).
Most of the all-inclusive places have a 'kid camp' or whatever. find out what is going on each day. I would hate for you to PAY for dolphin encounter and then have them miss the day that they would have gotten to see dolphins for free!!!! But that depends on the resort - some of the kid clubs are better than others, depends on where you are!
Oh- also we didn't rent a car and were fine with taxis. The hotel picked up/dropped off at the airport and they also had a shuttle to Xcaret.
My SIL did rent a car and we didn't use it a couple of the days because we found other transportation that meant we didn't have to figure out/pay for parking etc. However, she did say it was safe and easy to drive and get around.
Have fun!