Buffy's mother decides that she want to "bond" with her daughter during a patrol.  When in the park she finds two dead kids on the playground.  The ritual-like killings starts Buffy mother on a campaign to catch the killers and against witches and other occult members.  Buffy, Willow and Amy are tied up in City Hall to be burned at the stake when Giles and the gang discover that these two children are in fact a demon that feeds of the conflict people cause themselves.  They are the kids that gave rise to the Hansel and Gretel fairy tale.