Thats bullshit, i've visited NCR and all other towns with characters that have over 50 child kills. It doesnt change anything except for the town where you kill the child, and even then the minus reputation deteroriates. Nothing to worry about here, just a cosmetic title.
Actually, it makes players shoot you on sight, watch out!
but the "evil" npc's respect you more giving you a 5% discount on "evil gear" (Raiders, Slavers, etc)
So if you kill any child, raiders/slavers respect you more ? (And what if it's their own child, by the way ?)