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!