Leaving aside the H&S issues already noted, you can go head to toe in the bunks but all depends on the kids ages and size. I think you would have to book reserved seats when you book.
The rooms are tight, but it is possible. A little of discipline when you are in the cabin will be required. In truth there is a lot to do on this ship, so aside from sleeping, I think you will not spend much time in the cabin. You could ask when you get on board if they have a larger cabin free (late cancellation)
My only other bits of advice are:
- Pack togs for the on-board pool, small but the kids love it.
- Book the buffet restaurant - Le Flora, the fancy one. Let your holiday start on the boat rather than when you get off.
- Pay in € on the boat, they may still encourage you pay in £, but the rate is mad.
Another possible alternative is for one adult and child to travel by ship and the others to fly in on the cheap to somewhere like Nantes (easy in - easy out). We have done that in the past due to work commitments and it works fine.
Have a great holiday which every way you travel.