At the 1:38 or so, minute of that video, you can clearly see the lady leaning over the handrail, that is waist high for most, and at least a foot or so from the actual window or wall of windows.
I think the kid wanted to see outside, and he put her up on that rail or even over that rail, and lost his grip. I think the authorities had to decide to arrest him, as the family, and their attorney, have put so much misinformation and denial of responsibility, there was no choice. Plus, with a law suit on the horizon, with the cruise industry a big part of that Island's economy, I could see why the arrest was now made. Reasonable man action is the issue.
I still have so much sympathy for that family. Losing a child is horrific.
Accidents happen, when people do not understand their actions cause harm, or even death, like here. Even good, upstanding, loving people does not mean, they understand the risks, until something bad happens. And denial and tyring to understand what happened, makes a mind try to rationalize. I also feel so sad for the grandfather. Nothing he does will bring that little girl back.