No.
These things rarely, if ever happen. And when they do they are usually benign like the woman who gave out Steel Wool to kids she thought were too old to trick or treat. The reports of candy tampering are almost always debunked or proven false later on or the work of an inside person, meaning siblings.
http://en.wikipedia.org/wiki/Poisoned_candy_myths
http://www.snopes.com/horrors/mayhem/needles.asp
It is simply to easy to detect the tampering, as the package would have a slit in it that would be immediately noticeable for razor blades. Contrary to what the nightly news would like you to believe, there are very few real sickos in the world. Even if this were to occur, the damage would be limited to cuts and scraqes of the mouth tissue, and that always heels quickly.
What you should really be worried about is the effect that the sugar and fat have on the children.