In Children of Dune, Leto and Ghanima are nine years old. Because of the melange ingested by their mother, which causes unusual effects for the Atreides bloodline, the twins are "pre-born", meaning that as fetuses they had been awakened to consciousness and to their genetic memories; they had been born as fully matured human beings in the bodies of infants. At the start of the novel, Leto is not prescient to the degree that his father Paul had been, but he senses the test his father had faced — to embrace a prescient vision of the universe is to set the universe on that path, a terrible responsibility that comes with terrible power. At the end of Dune Messiah, Paul had forsaken that responsibility by walking into the desert — his time as the Fremen messiah had shown him that he was not strong enough to be messiah/tyrant to the universe. Leto believes that he must face the same test.
At the same time, the Imperium created by Paul is ruled by his sister Alia Atreides as regent. The horror of the pre-born—the reason the Bene Gesserit call them "Abominations"—is that they are easily possessed by the ego-memories of their ancestors. When Bene Gesserit awaken to their "other memories" in the rite of the spice agony, they are adults with fully formed personalities, and can withstand the inner assault of their forebears; the pre-born have no such defense. Like Leto and Ghanima, Alia had been pre-born, and she succumbs to the pressure under an intense dose of spice. Among her ancestors is the Baron Vladimir Harkonnen, still hungry for revenge against his enemies, the Atreides. Alia is possessed by him, and unconsciously turns against the Atreides empire, plotting to kill Leto and Ghanima and to tear down the Imperium in a bloody civil war.
Independently, Leto and Ghanima both solve the problem of the pre-born. Leto constructs his own personality out of an executive committee of his ancestors; with all (the important ones) inhabiting him, none can possess him individually. Following an assassination attempt by House Corrino, Leto disappears into the desert leaving Ghanima behind. As part of Leto's plan, Ghanima hypnotizes herself to believe that Leto had been killed. The intense mental discipline needed for this self-deception builds a safe haven in Ghanima's mind for her own personality to safely develop, with the memory of Chani acting as a guardian.
Leto finds Jacurutu, a sietch that has been forbidden to anyone by Fremen law. There he faces the test his father had refused to take, and embraces prescience, its visions, its attendant power, and the terrible price it will extract — to follow his vision, Leto will become a symbiote with the sandworm, setting the universe on the Golden Path, a future in which humanity's survival is assured. After consuming massive amounts of spice, he allows many sandtrout to cover his body, the concentration of spice in his blood fooling the creatures: