Can be books, songs, expressions etc B R G (7 6 5 so guessing its green - lol) T C O G K (3 8 2 5 5) T R C (3 3 8) - got an idea for this one but not sure
I've just looked this up, and she's written another book which fits - The Chimneys of Green Knowe - but I think the Children of G K is her most famous children's book and probably the one they are looking for.