I don't know about the locks on the balcony doors. BUT...there is plexiglass on the balcony and the balcony is a little higher than waist high(I'm 5'8"). There are usually 2 deck chairs and a small table where you might sit a beverage. Way too much temptation for little guys to climb and look over!
In my opinion, I would be worried constantly about a balcony with children that age. They aren't old enough to know the danger and if you took your eye off them for just a second, you would have a disaster.
Probably you would be ok at night, but my fear would be getting dressed. The cabins are small and in the hustle and bustle of getting dressed, unpacking, etc, you are more likely to not be as watchful as you would need to be.
Of course, my knees get weak when my husband is standing on the balcony and leans over to take a picture.
I'm sure there are parents that have had a balcony with small children, maybe they will have some suggestions or ideas.
The little guys are going to have a great time!