They know all of your children by buying that data from others. If one of your children is in middle school and one in elementary, they know which one is in the car by which location and time you picked them up.

If you regularly go to a soccer field after picking up the kid in middle school, they now know that that kid is interested in soccer. They can sell this data to advertising companies who will use it to show your family soccer ball ads, cleats ads, tickets to soccer games, etc.