When I was in elementary we had a field trip to an exhibit called "Body Worlds". It was actual human corpses posed tastefully. A few of them were pregnant and you could see fetuses inside.
Could be from a 8-9 year old or it could be from an animal like a dog or many other things. Mammals share the same amount of vertebrae as us, an extreme example would be giraffes necks, they have the same amount of cervical vertebrae as your neck, the shape of them is different. So find an animal with the right sized spine and go nuts claiming it's from a child.