Couple of thoughts - just treat the window as a square but if you want to keep the uniqueness of the window showing on the inside (and dont mind not opening it) then try this. Get some wire, wood or pipe or something that will go in the shape of the window, then some fabric (I'm thinking strips, thin sheer fabrics to let light in, whatever you want here) then start at the frame, looping with another from the other end then going back to the frame. Wood frame, you can staple or tack it down. If that doesnt work very well, get a small ring large enough to hold all the fabric through it, but not so large that you have a spot of bright sunlight (unless you want that) then simply thread your strips through this and spread it evenly around the circle. 100% customizable to fit your daughter's room decor!!
Hope you have a great & merry Christmas as well!