I fully agree that if this was a play area then the window should not have been open, I guess the question is who opened the window, was it a guest who was there before and walked off and didn't shut it. This area would have been converted from the ships original design and into the play area. I have seen these windows on cruise ships I have been on and they would be impossible for a child to open so to me that would indicate that an adult would have opened it and I think it unlikely a ships crew would have done it. I do think it is highly risky for a responsible adult to hold a small child who is most probably wriggling and squirming in their arms up to and slightly outside of an open window 10 storeys off the ground. Remember the Michael Jackson affair and the outcry when that was made public. That will be why the police are haunting the grandfather as they will believe it was an unsafe procedure.
Just my thoughts, don't shoot me