First of all, stop worrying. Yes, it's the lowest passenger level and the furthest in the rear. But, that doesn't mean it is a bad cabin or that they won't be able to sleep. They may feel and hear a low vibration, but that actually might help them sleep. I would keep them up as long as possible, dancing, go to the late shows, have a few drinks every night with them, in other words have fun with them and enjoy them while you can. They would feel bad if they knew you were worrying about them. I don't think that ship is full so if it doesn't work out, you can ask for a cabin change. They should be able to accomodate you.