He must have seen it to have mentioned it in his statement. It's only a fact he could know from seeing it happen IMO.
So he either did it during daylight or turned on a light in the room. I imagine there are window coverings to block out light in the children’s rooms. A nighlight would not show much. To me, a hall light would be too far away to show much in terms of color in a dark room. But maybe, then.