The warm skin tone and the bright colored makes this shot very interesting.
The subject seems to be framed within a frame by the way the window is open and helps to focus ones eyes on the girl. The horizontal elements of the siding on the house also bring your eyes to the subject.
I can't help also to like the reflection of the outside on the upper part of the window. Your also using the rule of thirds.
This picture works incredibly well on so many level. This is like a lesson in compositioning. If this is your first candid I can't wait for your next effort.