It will debut over Christmas this year, reports contactmusic.com.
Actresses Jennnifer Saunders and Meera Syal will also star alongside 12-year-old newcomer Billy Kennedy.
Walliams said: "We have put together a dream cast for this, the first of my children's books and for many, still my best. It is a funny and poignant story about what it is to be different. I hope it will make everyone who sees it, young and old, stop and think."
Reports suggest Moss will play herself in a dream scene.