The Blackberry Elf by Mary Louisa Molesworth

Nora and Hilary were staying in the country with their cousins. It was a new part of the world to them, for their own home, though not actually in a town, was not far from one, and therefore far less rich in wild flowers and mushrooms and blackberries, and all such delightful things than the _real_ country place where these fortunate cousins lived.

Had it not been for the newness and the freedom of it all, they might have found it a little dull, for there was only one child in the family at all near their ages -- Nora was eight and Hilary six -- and this was a boy of seven called Cecil. Cecil was very much younger than his brothers and sisters, and seemed even younger than his age, for he was small and delicate, and very quiet. Hilary, a great big strong fellow, seemed much older; indeed if you had seen the two together you would certainly have guessed that Cecil and not his cousin was the, so to say, town-bred boy. Cecil had never been so happy in his life as since the two little visitors had come to stay with him. They seemed to find out all sorts of new things that had never struck him before; pleasures and interests springing all about and close at hand which he had never thought of.