It’s Called Art, Look It Up
This story is too freaky to be true, but we’ll give the OP the benefit of the doubt. One time, they babysat for parents who were highly successful in the art world. By successful, we mean they had multiple houses and fancy cars, according to the OP. This is like striking babysitter gold, but there was one tiny hitch.
One of the kids they were watching had a habit of storing dead baby mice in jars of formaldehyde. At eight years old, we were thoroughly disgusted by mice, but this young girl was actually taking the deceased fetuses of mice and preserving them for some nefarious purpose.