Those levers take quite a lot to open, plus sliding the door would also take a lot of strength as well. I'm not sure a child that old has what it takes to unlock and open those doors, but I guess it's possible.
I was told they have some type of childproof device they can lend you to insure the door cannot be opened. Not sure exactly what it is and I've not personally seen one, but this was passed on to me by one of our clients who had a small child.
I have seen the kind where you insert a small rod in the top frame, I'm just not sure if it was on the Oasis-class ship or not.
Have your agent call RCCL and inquire about this for you.