Once by Morris Gleitzman

Once, I read a book from start to finish without putting it down. Once, I read a book that made me laugh. Once, I read a book that also made me cry. Once is the story of Felix, a young Jewish boy with a talent for storytelling - inspired no doubt by the fact that his parents once owned a bookstore. Set in Poland, 1941, the world is in turmoil in the midst of World War II - not that Felix realizes, safely tucked away in an isolated orphanage by his farsighted parents in 1939, in the hope that by disguising him as a Catholic orphan, he will escape persecution from the Nazis.
Felix waits for three and a half long years for his parents to return for him, all the while believing they have simply gone on a long trip to find out why the source of Jewish books for the store they own and run has dried up. When an entire carrot appears in his soup (a small miracle) he is certain it is a message from his parents, telling him they are returning for him. When they don't, Felix sets off by himself to find out what's keeping them from coming back. What follows is the tragic tale of a small boy, who, armed with an active imagination and his unwavering faith in his parents, is a shining light in a story black with the horror and depravation of the Holocaust. The tears you will shed when you read this book will dry, but it's message of sacrifice and hope will stay with you forever.

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Then by Morris Gleitzman (the sequel to Once)