Number the Stars and The Giver, which are moving and thoughtful. Instead, The Willoughbys is more of a parody of classic British children's books about children in a family, such as Mary Poppins.
There are a lot of things going on this book, most of them (I am sure) making fun of some children's book or another that I haven't read. The kids find an abandoned baby on their doorstep, to start off, and leave it at the house of an old, crazy hermit who lives nearby. Their parents hire a nanny (Mary Poppins?), and then the parents decide they need to sell the house and get rid of their children, so they leave on vacation. At the same time, the kids decide they need to get rid of their parents. The only question is who will succeed first!
Meanwhile, there is the side story of the crazy hermit and his lost son. Naturally, the Willoughbys' story ends up intertwined with his and the baby's, with the result of much amusement. Definitely not a serious book, but it sure is a lot of fun, especially if you grew up on the sort of children's books this one is making fun of!