I've ruled that children in my world are TN, unless their race has an innate alignment descriptor. The soul doesn't start "keeping score" until around the onset of puberty; at that point, major intentional acts can begin to make them drift away from the center. As age increases, the "scoring" system gets less lenient, until finally they reach adulthood and everything they do counts toward alignment.
This leads to some interesting effects in the gameworld. For instance, thieves' guilds (and hidden cults) recruit children not just for their small size, but also for their ability to bypass many traps and effects triggered to alignment. An antipathy spell or glyph of warding that wards against evil will be no protection against a sneaky 7-year-old. A child thief who hits puberty is like a boy soprano whose voice begins to change; no matter how much talent he retains, he's lost something he will never get back. (I toyed with the idea of extending that correspondence even more, saying that preventing physical puberty would prevent alignment change. But I ditched that concept, because I didn't like the idea of a preocious evildoer castrating himself and staying TN for the rest of his life.)