Went to Cancun in 2006.
If you must go, Xel Ha is beautiful. Take the tour bus and pay extra for the deluxe bus (if that is an option) with air conditioning and spacious seating. We enjoyed the ride, while the other bus complained about how miserable it was.
Also, I recommend swimming with the dolphins. Your kids will love it, as will you. one of those things that you dont have to do but reflect back on as a good call. Very fond memories and pictures of my family doing such. Cost about $1000 for 5 of us. Life time of memories.
I don't recommend the Mayan Ruins (Chichen Iza) with the kids. Very interesting but a bore to the kids. Plus hot as hell. They will not like it and will complain.
Stick to the legit tours. Don't try to save money by dealing with a middle man on the street. Just do it from the hotel. They have tour reps all over. Much Safer.
Do not take the local buses. Where I stayed, they had a sort of shuttle service. Had to share with others and wait a little but keeps you out of the day to day riff raff.
Stay within the resort beach areas. When going into town, should go without saying, stay in the tourist spots. Don't be lead a stray to save 10 bucks.
Finally, understand that Mexico is a mess. Everywhere has it's dangers. I actually canceled a family trip to Acapulco because there was recent shootings within the Hotel Resort areas. So, my best advice if you must go is keep everyone close, no wandering off, and stick to the tourist stuff. No mom and daughters do this, while father and son do that. Not saying you cant go swimming in the ocean while the others lay out on the beach, wouldn't split up while shopping or in town.