My family of 4 was in a cat 10, and we were comfortable with the amount of room. I think the cat 12s only sleep 2-3 people, and some cat 11s don't take 4 either. The cabins that sleep 4 have a queen size bed that can be split into two twins (might be good for you and your mom), and a sofa that flips into a twin bed. A bunk bed pulls down out of the ceiling for the 4th person. You'll probably have battles between the kids for the rights to the top bunk.
The Cat 5-10 rooms are all the same size cabin. The only difference in this category range is location and window/verandah. I won't say we had room to do gymnastics or anything, but we didn't have to turn sideways to fit in either.
Enjoy!