Lindsey, I think Sue was referring to her children’s books. The Mitford Series, while very good, is a bit much for small children. They are very long and do touch on some topics best suited for older readers.
Her children’s books are The Trellis and the Seed, Jeremy: the Tale of an Honest Bunny, Aunt Fannie’s Hat, Violet Comes to Stay, and Violet Goes to the Country. The Violet books are indirectly related to the Mitford series, as the Cynthia character is an author of a series of children’s books about her cat Violet.