Dropping the Kids Off is one of the many Side Quests in South Park: The Stick of Truth. To start the mission you need to talk to Mr Hankey.
His home is in the sewer, head there and he tells you that his wife misplaced their children. You need to search the sewers to find the three children, Simon, Cornwallis, and Amber.
Amber is the closest. Head to the left of the house and climb up a ladder, then walk back behind the flow of sewage. Turn the knob to shut off the water and then take the ladder down to find the first of three kids.
Simon: Climb back up the ladder, now go all the way to the right and then go down the ladder. Head back to by Mr. Hankey's house. Go all the way right down here, then shoot a ladder to bring it down. Then walk to the left, and shoot a knob to stop the flow of sewage, and go all the way to the left.
You will find a second kid behind a grate here. Use your weapon to kill the small rats around, and then it will call a big mama rat. Kill the rats to save the kid.
Cornwallis: Go all the way to the right until you see wood pallet leaned against the wall. Smash it to expose a lever. Pull it, and then go down the ladder it opens.
Fight off the hobos, to rescue the last kit. Once you have rescued all three go and talk to Mr. Hankey now to unlock him as a summon.