I would have to agree with Nancy. Royal Caribbean seems to cater more towards younger adults with just the right combination of freedom and activites. My son loved Royal Caribbean when he was that age and he hates anything organized and would never go to a structured event.
They have a "tweens" club where they can hang out and their counselors are more like "buds" than counselors.
I guarantee they will have tons of fun no matter which ship you choose. They obtain a kind of freedom on a ship that they just can't experience anywhere else.
One thought, with two young ladies their age, you might explore getting walkie talkies for them to each carry at all times. That way if they need you, each other or you need them, you or they are there in a heartbeat.
Have a great cruise.