sit for someone
1. to care for someone in the role of babysitter. I sit for Timmy sometimes. I like him. He's a good little kid. Mary doesn't sit for anyone anymore. It leaves no time for a social life.
2. to serve as a babysitter in someone's employ. I sit for Mrs. Franklin every now and then. Ted used to sit for the Wilsons.
3. to serve as a model or subject for someone, such as an artist. She sat for the portrait painter every day for a week. she is looking for someone to sit for her so she can develop her skills.