From what I understand no one under 18 is allowed to occupy a cabin alone. HOWEVER from age 16 they are allowed to be unattended (not have an adult with them at all times) so you could book 2 cabins - 1 with 3 and 1 with 1 person (both have 1 person over 18 per cabin) and allow DSD to be unattended in one of the cabins while you, DH and DD share the other one.

We had to book 2 cabins (next door to eachother) becuase we have 3 kids, so DH has a cabin of his own and I am sharing with the kids. I am not allowed to leave the kids unattended but the lady on the phone said that if, for example, they fell asleep waching a DVD and I didn't want to disturb them it would be ok to go into DH's cabin