Generally, the longer cruises will have fewer children. That said, there will still be more kids than you'll find on other lines. Repositioning with the lowest number, typically. B2B cruises won't cut the kid load as each cruise will have lots of kids (but not the same ones).
Any cruise out of FL will go to either Castaway Cay or Lighthouse Point (private islands).
I prefer the classic ships (Magic & Wonder), but have cruised on both Fantasy and Dream. Haven't done any of the newer ships.
My favorite longer cruises on DCL were Southern Caribbean on the Fantasy, and Panama Canal, as well as Med/Westbound Transatlantic both on the Magic.