I have to admit that the idea of an Immortal kid being adopted actually came to me sometime after I read "Square Peg". Then the idea was that the SGC and the NID have a basic idea of the Immortal life-cycle, but the possibility of Immortals stuck as children forever hadn't occurred to them yet. So the NID decides that the best way to get loyal Immortals in the long term that no one can really object to is to have the top brass adopt pre-Immortal children and to raise them so that they are mentally conditioned to be loyal to their family and the family's beliefs. They manage to find an Immortal who seems to be willing to do anything for money and/or heads and convince him to point out pre-Immortal children. Only, the Immortal is actually trying to get information and/or control the NID (possibly one of his students was taken by them?) and might be one of Methos's students. So the Immortal recruits children who are already Immortal to be adopted and spy on the NID, gathering information about important contacts and locations and about how to take down the NID if necessary. The con couldn't last long because the children wouldn't grow any older, but they'd want to get out before the NID finds out about Kenny and others like him anyway. Depending on the personalities of the Immortal kids and how dangerous the NID are, the kids could plant explosives at the homes to kill everyone and place the blame on an enemy of the NID. (Does the NID have enemies other than the SGC?)