Like almost every cruise line out there, the older the ship, generally the less there is for a kids program, and kids areas. Check the itinerary (7 day and less will have better kids programs, 8 days or more generally lighten up on the kids, because it usually has less families traveling on them).
The newer ships of Carnival, NCL, RCI all have wonderful kids programs and of course, your kids will make the most out of what is put in front of them. Check into each ships deck plan and see how large the childrens areas are if that concerns you.
It may also be to your advantage to look at the hours of service for the kids clubs. Some lines don't have them going late into the evening, but do offer "sitter" services for a nominal charge. Others, like
DCl offer later activities for the kids.
Personally, I would choose any one of the three mentioned above for families with kids - But remember to stick with the newer ships. I have traveled on all three a number of times, as well as DCL. For the price, I would choose one of the above three over DCL anyday. They (all four) are extremely comparable.
So, for the price.....it's your decision.