No the sliding doors do not have a removeable key lock. It is simply a built in system. If the kids are big and smart enough, they could open it with almost no problems. The doors are a bit heavy though, so not sure if this helps...
We travel with 3 kids, (ages 12, 14 & 16) and always get 2 rooms.
We do a corner aft balcony for me, hubby and 12 yr old daughter, and do an inside cabin (across from us) for the two older boys. We would never do a balcony cabin for them. Afterall kids will be kids at any age and we don't want to take any chances.
I would suggest checking out the current prices on an inside cabin (only if the kids are old enough to be a door or two away). You will save a lot more money than the other balcony and a lot of peace of mind too!