So I was talking with a friend earlier, and we came up with an idea of how Sarah was able to give the children powers, and how she chose her subjects in the first place:
Basically, the children are descended from supernatural monsters, however they wouldn't know that part of their family. While these powers develop over time, they only turn into something that can actually be used by the person after death, since it takes longer than a human lifespan to develop. That is, until Sarah found out about this and developed an injection that speeds up the process, however it only works on teenagers.
Sarah tracked the children by the slight genetic differences that they carry because of their ancestry. Sarah wanted to use this discovery to force the children to destroy the world, however I believe that the children have other ideas....
(Just gonna post here so this doesn't die. If it does, do y'all mind if I borrow the plot to write a book off of it?)