The exact origins of the Church of the Children of the Atom is unknown, but it seems to have already existed as far back as Megaton's construction, as Manya mentions that they were instrumental in getting the town built. The citizens of Megaton do not mind them residing there as their technologies and workforce were needed to build the city, in exchange for it being "built around the bomb."
By 2287, the church's influence has grown over into the Commonwealth, and even as far north as Maine, with Children of Atom being active on the distant island. The Children of Atom residing in the Commonwealth are also far more fanatical, with most members opting to attack the Sole Survivor on sight. However, those found in the Crater of Atom are passive during the quest The Glowing Sea.
“The Church of the Children of Atom believe the war of 2077 was actually a great holy event perpetuated by their god, Atom.”— Fallout 3 loading screen
The Children of Atom believe that each atom contains within it an entire universe, and when an atom is split many universes are created. Therefore, instead of seeing the Great War as destructive, the Church believes it was a creative and unifying holy event. The Children of Atom also see death more as a celebration of life and unification to Atom through "the Glow." "The Glow" is the earthly embodiment of Atom and the Children of Atom's direct connection to Atom. The Children of Atom view the ghouls as "Atom's forsaken" or those who have unfinished business and, for denying the ghouls their unification with Atom himself, Atom touches them with "the Glow" and asks that they spread the word of Atom to those around the world. Those ghouls that refuse to accept the "gift of the Glow" and refuse to spread the word of Atom are doomed to becoming feral ghouls.
One sign directly below the Megaton church reads 'Local Cult', which means that either they acknowledge themselves as a cult, despite the Confessor's arguments, or that they are unaware of the sign or the meaning of the word.
Unlike the mostly peaceful Children of Atom encountered in the Capital Wasteland, members of the Commonwealth chapter will aggressively attempt to "convert" all nonbelievers by spraying them with lethal amounts of radiation, including the Sole Survivor. Their indiscriminate hostility often leads to three-way fights between the player character, the Children and whatever else happened to cross their path, be it animals or members of other factions like the Gunners or the Minutemen. The only Children of Atom that can be talked to are the ones found in the Crater of Atom and Henri inside the Sentinel site, and even those boast a fairly volatile temper towards non-believers.
True to their beliefs, church members in the Commonwealth rely almost exclusively on radiation weapons in combat, mainly the very dangerous Gamma gun and the even more devastating Nuka grenade. They also make liberal use of nuke mines to secure their strongholds, making assaulting those an even more hazardous undertaking. While the Children usually only wear rags they boast a significantly large health pool compared to similar NPC types. In addition, the amount of damage they can inflict is staggering, especially given their nearly unerring aim with their Gamma guns.
For a reason that is not yet scientifically known, many of the Children of Atom (and an unknown amount of other people, e.g. Grand Zealot Richter discovered this prior to joining the Children of Atom) are immune to the adverse effects of radiation and live freely in high-radiation zones like the crater of The Glowing Sea without having to suffer from ghoulification or other mutations and some may even be able to remove radiation from others just by being near them. The Children of Atom claim that this is Atom's "gift" to them, but they never elaborate further on the matter of their adaptation or how inductees gain such a "gift".
Some of the Children of Atom do use anti-radiation medication, referring to the act of using it as 'scouring'. They do so either because they need it (since "Atom is picky about sharing His blessing"), or out of religious motivation as a form of penitence: 'Scour oneself and embrace the Glow anew'.
A small group of Megaton's Atom followers were directed by Confessor Cromwell to travel north, they settled on the Island on Maine. Eventually taking up a stronghold in the old Submarine base, thus creating the Far Harbor branch of followers.
So far the Children of Atom have been encountered only on the East coast.