Kate will make her cameo in the new TV series this Christmas and she will be joined by Jennnifer Saunders and Meera Syal along with 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." 
It is thought that Kate will play herself in a dream scene. 
The Boy in the Dress will be released by the BBC this winter.