This seems to be a new behavior that I don't recall ever happening. My 10 year daughter showed me some very inappropriate background images being displayed while she browsed the kids library. The background images were from movies that she doesn't have access to. So if she doesn't have access to those movies, why are background images from those movies being displayed in the kids library? I recall that before only images from movies that she had access to were shown in the background while browsing.
The only thing I have found is that I can go into each client as the kid user and disable backgrounds, but this needs to be done with many different clients and there is nothing stopping the kids from turning the backgrounds back on.