William S. and the Great Escape — Zilpha Keatley Snyder

William S. Baggett is a good kid trapped in a really awful family. William has his running-away money ready to go, he's just been waiting until he's older than twelve to leave. When his big brothers flush his sister's pet guinea pig down the toilet, she insists they leave now. And take the two littlest Baggetts with them. So they head out in the middle of the night, ready to escape to their aunt Fiona's house.

Unfortunately the trip doesn't go exactly as planned. It's not so easy traveling with two little kids, and some help from a lonely rich girl makes it even more complicated. Will they ever make it to Aunt Fiona's? And if they do, will she let them stay?

This is the story of four children who learn that sometimes you have to run away before you can find your way home.