It's most likely not a true story. Such names are often employed in fictional literature or media to draw in readers/viewers and tell an imaginative tale.
The key elements in this 'little girl in the window' true story would likely start with the girl. Her expressions, what she was doing at the time she was seen through the window are crucial. The window's characteristics matter too, like if it was a large or small window, clear or dirty. The context around the window, such as the surrounding scenery or the state of the building it was in. And then there are the possible observers. Their motives for noticing the girl, and what they did after seeing her can be important aspects of the story.
I'm not entirely sure which specific 'little girl in the window' true story you are referring to. There could be many stories with such a title or description. It could be about a little girl who was witnessed in a particular situation through a window, perhaps something sad or heart - warming, but without more context, it's hard to say exactly.
In fact, 'Girl in the Basement' on Lifetime is indeed a true story. It brings to light a harrowing real-life situation and the struggles faced by the victim.