You'll be held responsible if the plume hits anyone other than Grelod. But it might attack others, and you'll be held responsible. It will probably also attack Grelod if she's in range. I'm unsure whether you'll be held responsible for its actions, as the ash guardian spell is implemented differently from standard conjuration spells. This is challenging because Constance only has 30 hit points. Attacking any of these people is a crime.
Children are immune to damage, but Constance is not.Grelod has 1 hit point, so any damage whatsoever will kill her.Grelod has to end up dead by some means.Let's talk about how this quest actually works internally: