What a great question and certainly a timely one with the recent holiday. You're right, Julie. Every parent (even non-parent adult) has a different opinion on this topic.
From a dietitian's perspective, I think moderation is the best method with a small portion each day as a treat. Why not serve a piece or two with dinner each evening? Or pack one with their school lunch? Though before you even begin with the daily sweet, sit down with your child and go through their bag of goodies. Have your child pick out their favorite kinds and then remove the ones that are not quite so favorite.
Once your child has removed their favorites, you can discuss the idea of donating those pieces of candy to troops overseas through Operation Gratitude (www.operationgratitude.com ).
However, if you have your heart set on letting your kids just dive in without relent, one day is not going to make an incredible difference in their long term health (as long as you do not let them dive in every day after that). Your child may surprise you and stop after a handful of pieces, wisely opting to save the rest. Though if they do seem to be indulging without relent, it may be wise to help your child understand that eating to much may cause a belly ache.
Best Witches :)