With a family that includes young children, I would def recommend PM over Cancun. Now, having said that, I have to admit - I have never stayed in Cancun proper (just visited)! But - from what I have read it is crowded and more geared for people looking for a party. I have a feeling, you would prefer something a little less crazy and a lot less big.
Moon Palace is not in"Cancun" but it is way toooooo big, IMO, for a family. I have stayed there on a couple's vacation.
I stayed at El Cid last year with my family and it is very nice - just not one of my favorites. No real complaints though, other than I didn't really like the two breakwaters by the beach - blocks your view. It was clean and the food/drinks were on par with other AI's of this level.
OCT - fabulous - loved it. Beautiful, good food, great drinks, fun (not too crazy/party) atmosphere. I did not stay there with my family, but there were plenty of families there. Going back again in October for another couple's weekend. This place is so great - trying to decide if we will splurge on Privilege again...... love the Privilege beach area and a la carte breakfast so much.
Have not stayed at Azul Sensatori or NOW.
Another place you might want to check out are the Iberostar properties in PDC. It is big - not as big as Moon Palace - but the good thing is that they have a huge pool that intertwines the whole complex. My kids loved it. There are (I think) 3 resorts all together at different price points. We stayed at Del Mar (the lowest) and it was fine. When you're paying for 6 people to go to Mexico, you gotta take what you can get! :)
Have a great trip!