WordNet sense 1 (work or act as a baby-sitter):

baby-sit

WordNet sense 2 (assume a posture as for artistic purposes):

model, pose, posture

WordNet sense 3 (take a seat):

sit down

WordNet sense 4 (sit and travel on the back of animal, usually while controlling its motions):

ride

WordNet sense 5 (show to a seat; assign a seat for):

seat