You can either do this while progressing the main storyline (note its position in the achievement list) or you can start it at any point by doing the Strictly Business quest, or simply paying the guard out front of Paradise Falls 500 caps (if your speech skill is high enough).
If doing it during the main quest, speak with Mayor MacCready in Little Lamplight to accept the quest. From here, you will still need to gain access to Paradise Falls by doing Strictly Business or bribing the guard. Then you either need to do a long stealth-oriented sequence of events to break the children out, buy them for 2000 caps (1200 if you have high speech) or simply kill the entire town of slavers. You'll get good karma for the Town Slaughter since they are slavers, but if you have not already done Striclty Business you will lose your chance. So, be sure to do that before killing anyone in the town as they will all turn hostile if you kill any of them.
If you choose to do this as a side-quest before coming to it during the main story, the process of completing it remains the same, but you won't actually get the quest until speaking with Eulogy Jones. I would suggest just buying the kids. Once you have completed this and Striclty Business, feel free to kill the entire town for karma, human kills and experience.
See Rescue From Paradise for more specific info.