I was at the Iberostar Tucan July 23-29 with my wife and two kids, 8 and 11, and had a great time. We thought the buffets were fantastic - breakfast lunch and dinner - and I'm sure I could go many weeks without getting bored (I think there was more variety in a single dinner buffet than I see in my own home over the course of a year). We did spend several nights at a hotel in town prior to going to the Iberostar, and got to sample some of the Playa restaurants then. Our kids major complaint about staying in Playa del Carmen was that we had to walk into town for supper (kind of odd since the walk from our room to the buffet was probably as long or longer than the walk from the hotel to 5th Avenue).
I would not be worried about the resort being too picky about rules concerning kids in the adult pool, or rules about anything else for that matter. When I arrived at the room I actually read the long list of rules, and I can't say they really seemed to care a lot about enforcing them. For example there is a rule against bringing drinks from the bar into the pool area, but you can carry your drink right into the pool with you and no one will say anything (I didn't do this, but many others did). Similarly with rules against diving in the pool. They might enforce the "no going topless in the pool area" rule...I didn't see anyone breaking that one. The one rule I do wish they would enforce, though, is the one against reserving lounge chairs. (This rule the resort gives an entire 8 1/2 by 11 sheet). Most of the day, too many of the prime spots are taken up by the towels of absentee landlords. There are more than enough good seats to go around for the people in the pool area at any time, but not always enough for them, and for those who've gone back to their rooms but left their towels behind.
That being said, I can't imagine your family not having a wonderful time. We'd definitely go back to the Tucan again.
As far as souvenirs, I wanted to put in a plug for one little shop we went to. It's a small weavings shop just off 5th avenue on the water side, I think on 10th (It might have been 12th). Some women from Chiapas were selling a variety of hand made weavings - the type usually associated more with Guatemala, in various sizes up to table cloth size. We bought several items, paid their first asking price, and still felt like we got a great deal - even more so after I travelled to a shop in San Diego last week selling similar items from Guatemala and saw the prices there.