We've done a couple of NCL in the last 4 years and are Gold on DCL (and have a few RCCL cruises under our belts as well). If you're talking about younger kids and the kids clubs, obviously no other line compares to DCL. That said, my kids always enjoyed the kids clubs when they were in them on NCL or RCCL, so it was not a matter of the quality of the kids club/programs or the staff . The big difference, IMO is that on NCL and RCCL the kids clubs are not open continuously throughout the day like DCL. This really bothered us. The club would be open from 9am-Noon, close from Noon-2pm, open again from 2-5pm, and then close 5-7pm. They re-open again at 7pm and stay open until 1 or 2 am with a per-hour charge starting at 10pm, which we really didn't mind. That schedule may not be big deal to some - especially if you are early risers and have early/first seating for dinner - but it didn't work for us at all. On sea days, we usually sleep later and are rushing to get to breakfast just before they shut it down (usually 11am). So if we finish breakfast around 11:30am no point in taking the kids to the club at that juncture, where on DCL it doesn't matter because they are open all day. Now we have to wait 2 hours for them to re-open. Then, they close again at 5pm for two hours during the time it would have been good for the kid(s) to be in the club while Mom & Dad are getting ready for dinner (we always have later seating).
There was a lot about NCL that we did like. If I could create what I would consider the perfect cruise line, it would take different elements from DCL, RCCL, NCL, and even Carnival. I really liked the "Freestyle" approach to dining on NCL (which I know is blasphemous to die-hard DCL loyalists) and no formal nights. They also had some really good specialty restaurants like their Brazilian Steakhouse, Mexican, and BBQ restaurants. They also have a cool 24-hour Irish Pub-style venue that we enjoyed. On the negative side, we felt the service was a step down from the other lines in a few areas - particularly the stateroom stewards. Also, service in the MDRs tended to be rather slow.