They think that, the little angel will die, but she possess mutated power, with the soul/grievance of the deads children in her darkness, and darkness most of the time is related to death, curse and sickness, so i think the power can seal the thing inside it and two think can happen:
1 the sickness kill the soul of the children inside the darkness, reducing it's power and finally vanishing with the sickness when all soul are dead, so Leslie will be finally a "normal" child....
Or 2 he sickness is truly contained in her power and now her dark power contain also a power of sickness/rot/curse for all thing it's touch apart from Leslie.....