I greet them depending on what they're wearing. Usually I'll just say "Hello there, awww now don't you look cute/scary!" Depending on their costume. We get several bags of different candy and generally I'll let them each take a handful, but even though we had several bags this past Halloween, I realized at about the middle that if they all kept taking huge handfuls, we might not make it to the end, as we get loads and loads of trick-or-treaters, usually. I started just dropping two per bag in their little bags/buckets and they seemed happy with that.
They can never tell when it's dark anyway.