The Mystery Girl — Gertrude Chandler Warner

One warm night four children stood in front of a bakery.
No one knew them. No one knew where they had come from.

Henry, Jessie, Violet, and Benny used to live alone in a boxcar, but now they live with their grandfather. And they're working in a general store, to help out a friend of Aunt Jane's.

It seems to the children that the young girl who works in the store is behaving very strangely. She seems nervous and won't answer any questions about her past. Then a strange man comes to the store asking about her. What secret is the mystery girl hiding?