I don't really think that anyone "agrees" with the sentiment of killing children. However, in historical context, Psalms 137 was written during the time of the children of Israel being enslaved by the Babylonians. That Psalm may very well be merely a sentiment that raising a child to become a slave was not desired. There were also historical accounts of conquerors killing infants and children to try to prevent their "slaves" from advancing their races.
However, things haven't really improved over the centuries. How many have been murdered by conquerors or dictators? Even as late as prior to the invasion of Iraq, there were numerous incidents of Saddam's goons (and his sons) murdering people with whom they didn't agree. I'm not sure that one could call all the murderers as believers as many were or are likely secular in nature.
Weren't there instructions about killing disobedient kids in the Bible?