Man I had over 175 kids (not kidding). I purchased about 10 bags of candy and was out in less than 2 hrs.
The first year, I'd let them pick and they'd take too much.
From then on out I pick 2 pieces and give it to them.
I still have the occasional one that puts his/her hand in it and grabs a handfull, but since it's few, I don't say anything.