Peter, Phyllis and Roberta are the key characters in which work?

The Railway Children is a children's book by Edith Nesbit, originally serialised in The London Magazine during 1905 and first published in book form in 1906. It has been adapted for the screen several times, of which the 1970 film version is the best known. The Oxford Dictionary of National Biography credits Oswald Barron, who had a deep affection for Nesbit, with having provided the plot. The setting is thought to be inspired by Edith's walks to Chelsfield railway station close to where she lived, and her observing the construction of the railway cutting and tunnel between Chelsfield and Knockholt.

The story concerns a family who move from London to "The Three Chimneys", a house near the railway in Yorkshire, after the father, who works at the Foreign Office, is imprisoned after being falsely accused of spying.

There are tree children:

Roberta: Nicknamed "Bobbie", she is the oldest and most mature of the three children, and the closest in personality to their mother.

Peter: The middle-child and only boy. He is intelligent and resourceful, though at times rather insensitive. He considers himself the leader of the three and usually does take the lead in crisis situations.

Phyllis: The youngest and least mature of the children.