Stock photos had crossed my mind as a possibility as well, but I'd say it's unlikely to be previously deleted photos or from their Windows computer if they're seeing photos of kids they don't know.
All I am saying is that this is a very serious security breach if true, and everyone in this thread is taking a forum post at face value. There are a hundred things that could "cause" this, even if true.
The data appears to be improperly handled personal info. There is no evidence suggesting that the photos are from a "used computer" and the photos are directly downloaded via the iCloud for Windows client.
