When a group of foster kids accidentally summon a demon, the friendly, helpful ghost who already haunted their foster home must drive out the demon to save both himself and the kids, without exorcising himself.

Dig it!

Leaning towards:
“When inadvertently summoning a demon on Halloween, a group of foster kids sets out on an exorcism mission with a resident friendly ghost to reclaim their beloved home”

The “without exorcising himself” is also a good hook, although not in my version, it can be part of your ACT II fodder.

Keep going!