If one of your companions kills a child instead of you, you don't get this title.
Although you get the Childkiller title after killing one child, you'll need to kill three in Fallout or two in Fallout 2 before the people of the wasteland actually start regarding you as a Childkiller.
In Fallout 2, there are a few kids who increase your child kill count twice because of a script redundancy (and thus instantly make you a "real" Childkiller). These are Jonny in Modoc, the Slag kids and Curtis in Vault City.
Killing Myron in Fallout 2 will give you the Childkiller title despite him not really being a child, but the game regards him as one.
Behind the scenes
A new Vault Boy image for the Childkiller title was created for Fallout 2 but was never used.
This image was unused and the only Vault Boy image to ever be cut from Fallout 2. (I'm sure you can figure out why) I remember when I got the request to do a perk illustration for "Child Killer" that there would be no way to keep in from being offensive. I mean really! How do you make an illustration of "child killer" and keep it from being offensive? Anyway for some reason, I thought this was the least offensive way to do it. I have no idea what I was thinking. Even the designer who requested it realized it was a bad idea, so we fixed it. Looking back on it now, I can't believe I drew this.