Currently I am having the child as already being sacrificed to activate the Stone and its (as well as the 14 other childrens') desiccated body will be found around the stone. Once they start attacking the Stone, or take damage from the Stone's aura it will animate the children's corpses to fight back. Creepy, dark, and quite vile. =O
The older sister has been taken in an invasion-of-the-body-snatchers method and replaced with and aberrant clone and was responsible for taking the children there to the changeling who actually performed the ritual.
The PC's will have the option of killing the activator near the Stone or or a difficult skill challenge to allow the childrens' souls to be brought back via a special ritual. There is a lot at stake, children's lives... =)