I can't speak for Magic but I have been on the Dream and the water slides are identical. (The water park area has the dump bucket on the Magic, but the slides look the same.)
Never had any problem with unruly kids, the lines were always monitored when in operation. We tried the slides and all were fun. They were open most of the time. As long as the seas were somewhat calm then they were open.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013