My mum and I used to read the Enid Blyton books and series together when I was young; things like Famous Five, The Secret Seven, Magic Faraway Tree, Malory Towers, St. Clare's and Naughtiest Girl (the last three are series set in 1950's girls boarding schools, so might be more suitable for a girl rather than a boy though). I also remember Swallows and Amazons, Anne of Green Gables, Ramona, Five Children and It, The Railway Children, Stig of the Dump, The Borrowers, The Family from One End Street, Ballet Shoes, What Katy Did/What Katy Did Next, The Owl Who Was Afraid of the Dark, Paddington Bear, the Milly-Molly-Mandy series, Mary Poppins, Narnia, The Worst Witch series... wow, the more I think about my old childrens' books, the more of them I remember
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