I've done the Labyrinth Canyon float with the jetboat ride back with my boys when they were 8. *Great* trip, a little longer than you wanted. It's 56 (?) miles - a good 5 days at least. Might be a little chilly in March, but at least you won't have any mosquitos
Let me know if you have any questions.
I've also done the Ruby Horsethief - another great trip. That one has a bit of class II, mostly squirrely water, in the black rocks section. If you are in a raft you should be OK, with a canoe it can be dicey at higher water levels. That one can be done pretty easily in 2 days / 1 night, but I'd recommend at least 2 nights to get a hike up Rattlesnake canyon. My boys love that float too.
Have fun! If you decide to do Ruby-Horsethief I would be interested in joining you - that's my boys' spring break week (last week of March). They are 13, now, though, so we'd have teenager-vibes around at least part of the time