I found out recently that I have one living ONE door down from me. The crime for which he was in jail was aggravated sexual assault of a child. I felt the same as you.
I live within 500 feet of two schools and just couldn't comprehend how they could allow this guy to move in. I thought there was a law that sexual offenders could not live within 500-1000 feet of a school. I also thought the sexual offender and/or his parole/probation officer were required to go door to door within a certain radius of the house he was residing to inform the neighbors (which did not happen in this case).
So, the next AM I called the number listed on the registry. A very helpful woman gave me some info:
1) the 500/1000 feet rule is only under certain circumstances (high risk sexual offender) and this can even be waived by a judge.
2) A postcard (at least, in my state) is sent out to everyone in a mile radius of the sexual offender (I believe it was a mile, it may been more), but only if he is considered "high risk" and only if he released on parole or probation (so, not if he has served his whole sentence).
I totally understand people serving their time and living their life when they get out of jail. I understand that everyone should have that right, but that is hard to come to terms with when sexual offenders have such a high level of recidivism and your kid lives on the same street.
I guess the only comfort we have (and it is small to be sure) is that we KNOW about these offenders. We KNOW where they live and what they did. There are alot more out there we probably don't know anything about.