Yep, what [MENTION=12223]
Though, I do think putting the spell on the candy was a bit of a risk. Not every adult in Sunnydale would have had a child at that school, so there would have been some people who didn't have access to the candy, especially the parents of the babies in that hospital. A better way might have been to put it in the water, at least they could be certain that ALL the adults would be exposed.
They don't need to have a kid in HS to get the candy. When I was a kid and the school did fundraisers we were encouraged to knock on neighbors' doors and such to sell stuff. I'm told that's not near as common today because of paranoia that it might be some serial killer might open the door.
Yes, but that still might not reach EVERY adult in the town. Like you said, some people might not answer to strangers, some might not want to support a school they have no affiliation with if their kids are too old/young/go to one of the other high schools in the district, some might have been out of town when kids came to their door, and some might have been in hospital.
It seems impossible to me that the candy reached EVERY adult in town.