How I interpreted this:
The family I babysit for bought a third car FOR ME TO USE because it's convenient for them to have me drive their kids around in a car they own because I have no car of my own (or perhaps it's a $#+ box that's not safe for the kids). If one of the kids is, say, 12, then that can be his/her car in a few years. If one of the kids is 12, the parents Amy still need a baysitter because of everyone going different directions for sports, music and extracurricular activities.
The family I babysit for bought a third car FOR ME TO USE because it's convenient for them to have me drive their kids around in a car they own because I have no car of my own (or perhaps it's a $#+ box that's not safe for the kids). If one of the kids is, say, 12, then that can be his/her car in a few years. If one of the kids is 12, the parents Amy still need a baysitter because of everyone going different directions for sports, music and extracurricular activities.