I don't have any kids, but as others have said, it's just natral curiosity, and we all did it at that age.
I have however snuck around with plenty of little girls when I was 10-15 whose parents locked them up in the house to keep them away from evil little boys. We always got together to play or whatnot while their parents were thinking they had the entire situation under control.
I dated a girl in my neighborhood at the beginning of our senior year in High School, whose parents wouldn't allow her out of the house unless she was going to school. All because her mother got pregnant with her when she was 17 years old. We dated, had sex, and spent time over each others homes for an entire year and her parents never knew we even knew each other. They finally cut her loose after graduation where we dated openly for another two years.
In my experience, all the girls who were locked away were the wildest girls I knew, because they were determined to get out to see what their parents forbid them to do. I have vivid memories of this 12 y.o girl down the street whose parents made her play only in her backyard. She went to a private school and I didn't even no her name from the few times I ever saw her during the three years she lived in that house. She invited me to come play with her when I happened to be walking down the alley that day. I hopped her fence and we began kissing at her insistence, and only then did I learn her name. She really resented her parents keeping her away from the other neighbor kids. She was a textbook example of the perfect prim and proper daddy's girl.
I always wondered how she turned out as an adult.
I guess it makes parents feel better knowing they tried to keep their children away from the evils of the world.