I remember visiting Cancun during Spring Break many moons ago -- when I was single, had no kids and I ran a muck! If it's anything like I remember it, it's positively gorgeous and the people there are so hospitable. Now, if I recall anything from that time in my life, it's that Cancun can get pretty crazy around the 2 month span of Spring Break (anywhere from March to the end of May depending on what school you go to). If you have small children, I would recommend visiting around a different time. Of course, as I mentioned, things may be a little different.
I've never been to their Club Med and I've heard that it's VERY kid friendly. Heck, it may even be my next destination in the coming months.