The four Pevensie Children whom we first met in The Lion, The Witch and the Wardrobe: Peter, Susan, Edmund and Lucy. They re-entered Narnia when Prince Caspian blew on Susan's horn bequested by Aslan in The Lion, The Witch and the Wardrobe. They then returned through a door in the air created by Aslan.
In the book, Prince Caspian and all the Pevensie children returned to Narnia. Peter, Susan, Edmund, and Lucy were brought back while waiting for the train to go to school in England. They were brought back by the magic of Queen Susan's horn. It was said that if anyone blew Queen Susan's horn, help would come. Prince Caspian blew the horn in hope to receive help in the fight against King Miraz. Although they were expecting grown men and women, they were surprised to find the Pevensie's as children.
In "Prince Caspian," all four of the original Pevensie children returned to Narnia. They were waiting at the train platform in England, to return to school after the holidays, when they felt a strange tugging sensation that pulled them into a strange land, near the ocean. After much exploration, they discovered they were standing at the ruins of Cair Paravel, their old castle from "The Lion, the Witch, and the Wardrobe." Instead of returning as adults, however, they have returned as children, each the same age as they were in England.
When they later meet Trumpkin, the dwarf, they realize that Queen Susan's horn called them back to Narnia. Although Trumpkin is not impressed by the children, they soon demonstrate their competence and they continue on to meet up with Prince Caspian.
Later, at the end of the tale, Aslan makes a door to send the children and a few of the Telmarines out of Narnia. When they return to England, they are back on the platform, right where they were when they left.