We're going in late September on the 4 night with our DS, almost 4. We wanted to wait until he was potty trained so he could go to the clubs. We also are doing the 4 night for a "trial run" to see how he would do on the 7.
From what I understand, DCL is very kids friendly, even with the littlest kids. The nursery is supposed to be really nice and the clubs are "open" at certain times for parents to take their younger kids in to play.
It really depends on what you are comfortable with. We have no problem travelling with our son, but like Yellow Shoes, we chose to wait atleast until he was potty trained for the Mickey Cruise. If we have another child, I don't know if we would wait again.
If you are interested in more info, PM me!