I need help with a situation?

So, I'm writing a book and I need an 'interesting story' for my characters to use. There are three kids, two girls (16) and a boy (14). They are in the middle of the desert at night and a guy pulls up to offer them a ride. Except he wants an interesting story because he believes they are running away from home, while they kind of are, it's not for the reason he believes. The year is 2163 and there is a law prohibiting families to have more than two kids. Many sets of three siblings have taken to running away in order to start a better life for the youngest. One of the girls and the boy are related, but the other girl is just a friend. I just need an idea for this story that the guy wants to hear. Thanks so much!