This is JMHO, but I would
not book a balcony cabin with children this young. As you probably know, they're smarter than they look
and could easily bypass the locks and climb on the balcony furniture
!
I'd book an oceanview cabin so you have some sunlight. A cabin that sleeps 4 will have either 2 overhead bunks (along with 2 "on the floor" beds) or 1 bunk and a pull out sofa. Your Room Steward can furnish you with guard rails as/if needed.
If there's a pull out sofa bed you might want to put both of them there.
Have a wonderful cruise!