It is hard to believe that anything was pre-meditated. I don't think the grandfather would go for that. If I was asked to kill my grandkids, I would tell the parents to do the dirty work themselves. Also, the grand father did not bring the child over to the window, the child went there on their own and the grandfather chase after her. You can see this much from the video.
It was a tragedy and a poor decision by the grandfather to lift her out the open window. Not purposeful with any sort of intent, it was an accident. One thing for sure, you can see the grandfather first look out the window, and his head extends pretty far, I would think out of the window so he had to know the window was open. Also, when he lifts the child and swings her over the railing you can see it is done in a pretty quick motion, if he thought there was a window there I think he would have been more careful with her not to have her bang into the window.