The Times newspaper has quoted segments from an interview with J.K.Rowling and Stephen Fry which will be aired by BBC Radio 4 at 9am on December 10th. In the quotes JKR refers to a book which she has worked on which is aimed at younger children.

"There is another book that's sort of smouldering in a cupboard that I quite like which is for slightly younger children. There are other things I'd like to write. But I'll need to find a good pseudonym."

Source: Veritaserum