Unfortunately, it's not like children murdering people is a recent event. It's been going on since people have been here. According to the USDOJ Statistics, one-in-five murders of children are by other children. That one surprised even me. But there are accounts of children murderers in the hieroglyphics and ancient papyrus scrolls of Egypt and in the ancient chronicles of China, Mesopotamia and other places.
One of the most famous was that of Mary Bell of Newcastle upon Tyne in 1968 where she and a friend killed (at least) two toddlers, and scrawled their initials with razor blades into the chests of their victims. Another famous one on
this side of the pond was the "Boston Boy Fiend," Jesse Pomeroy, who spent nearly his entire childhood life torturing and killing other kids for fun and recreation. They sent him to reform school, but that really didn't work out.
You'll find Mary Bell and Jesse Pomeroy at #3 and #2 on
this list.
Other lists
here and
here. You'll find a few of them mentioned on more than one list.