Don’t squash the children! A sitter can be anyone who sits — in a chair, on the floor, or anywhere else — but it’s often short for babysitter. They don’t actually sit on the babies, though. We hope.
In addition to the person who sits or models for an oil painter, a sitter can also be someone who watches children as a paid job. It's short for babysitter: "Good news, guys, we're having the nice sitter tonight, the one who lets us stay up past bedtime!" The origin of sitter and babysitter isn't completely clear, but some experts guess the sit part comes from birds sitting on their eggs before they hatch.