The achievement is called Kindergarten Cop
You must find a radio and return a doll to the radio
Do not blow up the armory or you will void the achievement
During chapter 2 you will come to a jail with a combine in one of the cells you are supposed to go to the armory down the hall and blow it up instead turn right at the armory and head down the hall to the bedroom you will see a yellow radio you will hear a kids voice asking you to find her doll.
The doll spawns on the floor below where you fought all the resistance solders in the small room to the left of the manned turret on the bed when you return the doll to the radio the achievement will unlock.