I guess what I'm asking is why she would have seen it as appropriate at all to go anywhere futher than "the green." From personal experience, having lived in England as a child in a similar type of housing estate, when we played out like that we were always told not to go anywhere other than the green. You wouldn't get into a car and go off somewhere even if you knew the person, unless you were being disobedient. And April doesn't sound like a disobedient type of child.
So I wonder what did this person say to her to make her override the instructions she had probably been given, (and which were reiterated by her friend next to her), to make her get in the car.