UPDATE: The owner of the building wrote us:
"The little girls are not at risk. The plastic was put on the inside of the window, by the tenent over the winter, to cut down on heat loss. There is a large glass window there, you just can't see it. It's very secure."
Thanks to the owner for explaining the situation. And thanks to the neighbor who made the original report for being concerned enough to notice and follow up on what looked to be a dangerous environment for the two children.