The biggest fear I have in life is something bad happening to my children. I pray to God (literally, and I'm not a religious person) that if there are any "tragic" events he has in store for either one, be it illness, horrible accident, or any form of abuse, that he give the burden to me and not to them. I'm sure that sounds strange (or maybe not) but I can not imagine the alternative.
I think the worst thing that I fear is abduction or sexual abuse. I'm not one of those moms that overly protects her kids or shields them from the world. I let them be kids and carry the worries within. We can never be to safe and I occasionally check out a List Of Child Molesters in my area. Sometimes I look at their pictures in the hope that if I were to run across one of them I would put my guard up. Sometimes it's just a reminder of how cruel the world (or people in it)can be, and makes me hug the babies a little tighter.