Rene- Sounds like your intent on worrying about something! Sure you can worry about those things if you want to. But I wouldn't.
Failing over board? When you get on the ship, go to the top deck, take them over to the railing. Point to the water below and tell them the impact of the water from that height will mostly likely kill them alone, but if they should happen to survive, the ship would never know they fell overboard and keep going (Assuming the props didn't chew them up) then they would be treading water in the middle of the ocean. (At least until a shark came along). Then say, don't goof around near the rails, even as a joke. If they still manage to fall overboard it's Darwinism at work. (I say this with tongue in cheek of course
) A firm warning of the dangers of fallin goverboard should make sure they don't goof around.
Seriously though, they would have to be doing something really dumb to fall over board. The railings are very high so it doesn't happen purely by accident.
As far as your other concerns, you can always worry about something. The ship is a fairly safe place. Probably safer than going to a mall. The walkie talkies are a great option. Just make sure they don't over use them and distrub the other passengers with loud talking on the radios.
Make them meet with the teenage kids group on the first day. They will then at least be able to hang out in a group of kids which will help make them safe. Also arrange for meeting times (or use the walkie talkies) so that you can keep an eye on them.
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
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013