Hi Jaime,
I am from Cancun, I didn't born there but live many years on there and with my now 12 year old girl.
If it is true that Mexico can be dangerous (all countries have dangerous areas) Cancun is one of the most turist friendly places.
We had 2 very busy times with American and European tourist : Spring Brake and Blue's fest.
National tourist go there around Jun-Jul and April, some December but not as much.
So I think you would do ok if you are going now.
There is lots of things to do in there for kids, and most people speak English (with very heavy accents of course, lol).
I know you have to be careful with the water (drink only bottle water because our water is not the same then here), stay in the Hotel Zone, and use the bus instead of taxi, taxi are actually very cheap in there "for us" but they would charge you more if you are tourist (sad but true), and the bus are very clean and not expensive.
If you need more info feel free on PM.
Have fun
EDIT:
The places where you wouldn't want to go and i wouldn't go either are Mexico City (DF) and any of the states that are in the border with USA, but Cancun is relatively safe, Mexican goverment actually put effort to keep it save because is an important income of money and works for many Mexicans, many American and European are retiring in Cancun, in fact, hopefully we will retire there :0)
EDIT AGAIN: I am not sure where JL husband stay, but my now husband went to Cancun A LOT, when were dating (he is an American and we were dating for more than a year), he did got VERY sick ones, when I took him to my favorite restaurant, lol.
But he never got sick from brushing his teeth.
But I agree, every time you travel you have to be careful. I have been here for so long that even when I go there I am careful, lol.