I've considered becoming a teacher before, and one of the reasons that I didn't was that I just didn't want to be responsibility for the welfare of children. If something ever happened while they were under my care, I'd feel horrible if something happened, even if it wasn't my fault. Not to mention the liability in the eyes of parents, who can be less than reasonable when it comes to their children.
I've also refused to take care of children of friends and family, for similar reasons.
Finally, I've decided not to ever have children of my own, once again partly because I just don't want the responsibility, and partially out of a concern that I'd probably just screw it up somehow. Not to mention that the world is way overpopulated as it is, and it doesn't need me adding yet another person to the mix.