It's a children's book, probably written in the 70s - or even before.
It features a young girl, maybe around the age of 12 who discovers an abandoned baby.
The girl takes the baby to a safe place then looks after her, keeping her warm, fed and happy.
I think at some point the Mother of the baby returns for the baby - was the Mother a gypsy, or travelled with the fair, something like that.
The young girl I think had a younger sibling too, and the baby was a girl I think too.
I can't remember why the girl kept the bay as a secret, or how she managed to keep her so happy....I was fiction!
Would love to read it again as an adult....but have no idea what the name of the book is or the author.