I wouldn't let a 10 and 9 year old swim on their own. There isn't a life guard on duty and the kids pools tend to always be super crowded.
Wave phones are really the only way to communicate with each other. If say she is going to the aqua duck remind her she must text you when she is putting the phone in the cubby and she is to include the posted wait time and then call or text as soon as she is done. Tell her each time she rides she must text again. Besides that the phone should be close to her.
Just remember if you wouldn't let your child roam alone in say Wal-Mart or even WDW what makes you feel
DCL is any safer? Anything could happen do to another passenger or even crew. I don't want this to sound rude so just throwing out that is not my intention. My intention is to get you thinking about what makes DCL safe enough to let a 9 an 10 year old roam what is basically a floating city. Like others have said lab to club gives them freedom to move around but on the boat alone needs some serious consideration.