You might do well to consider bookmarking this handy guide to liability, as yes, you can be held liable for any injury to the little freeloaders. A few salient points from the guide:
When the porch light is on, trick-or-treaters are considered invitees; the homeowner is inviting them onto the property (though not for a mutual benefit). Because of this relationship, the homeowner owes the candy seekers the level of “reasonable” care that falls under Ordinary Negligence.
Even if the homeowner is not planning on scaring the kids, he/she must warn about the loose brick or cracked sidewalk, fix the hazard or protect the invitees from unrepairable hazards.
If the porch light is off, the same level of care is not required. The homeowner has not invited the kids onto his property to get candy.
It seems that, on the one hand, you're safer to not turn on the outdoor lights. Actually, you might want to consider turning off all lights and simply go into hiding for the evening. On the other hand, this approach invites an increased possibility of vandalism.