Kay, in my opinion, you can't go wrong with either ship. Both have great pool areas with water slides -- although I give the edge to Carnival Triumph here. Norwegian Jewel has a pretty neat-looking hangout for teens, including their own Internet stations.
Some other considerations: cabin size (Carnival Triumph is better here, IMO), dining options (NCL Jewel is better, IMO), and itinerary. Note that while NCL's Private Island is nice, occasionally the sea conditions prevent the Captain from stopping there as it is a tendering port.
Love2Cruise2, I haven't been on Norwegian Dawn, so I can't help with the kids program. Yet, I've found it's pretty common for the programs to allow pre-teens and teens to sign themselves in and out of the program on most lines. Of course, you can do what we do and not give permission for them to do so... even though your kids might not be pleased with this decision.