I do.
I dress up in my Incarnation of Death outfit and delight in their screams and wails. My best one so far is one little weezer who screamed, threw his candy at me and took off running. His dad didn’t catch him until the end of the block. Dad thought it was the funniest thing he had seen in years.
Share your best “Honey, I scared the shit outta the kids” stories!
We had a haunted house for kids in town to come to last year at the community center. I was hiding under a low shelf covered by black plastic. Because of this, I could see only the feet of the kids as they approached my position. Most of the time, I’d jump out (monster mask in place, of course) and get a shriek from the kids. Sometimes it’d be an older kid, who wouldn’t even startle that much when I grabbed ankles. Other times however, it would be a really young kid, who would immediately proceed to start crying. Oops.
Kid feet all look the same to me.
Not scaring them, but I’m going to have a big bowl of onions to hand out (and a bigger bowl of candy).
I wanted to give pickles and hard boiled eggs last year, but was afraid the eggs would wind up on my car or lawn! LOL
I want to do this so, so badly…
…but I’m 90% certain I’ll be working on Saturday. 
Maybe I can make blood and slime and bat smoothies at work?
Acid Lamp and I enjoy giving a good scare to the kiddies on Halloween, but try to make sure that they’re old enough to not crap their pants if we do it. One Halloween a few years back, I was still in college and we decided to make me a zombie complete with slit throat. Of course, that same evening, we had to pick up a friend from the mall and drop him off, so I go trawling through the mall with Acid Lamp guiding me around, as I was shuffling and keeping my eyes out of focus for effect. I scared more adults than I did kids or teenagers, but they all thought it was cool; I did, however, learn that there are a lot more superstitious adults in the area than I had thought before. Later that evening, we get back to the apartment, and I lay by the stairs that lead up to our first floor apartment, waiting for trick-or-treaters. I can’t see the kids coming, but I can hear their footsteps, so I wait until I hear them close enough, then spring up to a sitting position. [sub]You guys can see where this is going, eh?[/sub] An hour into the shenanigans and I end up scaring the bejesus out of a toddler with its parental unit. :eek:
Oops.
I never have, but man, my husband and his brother have some great stories. Pretty early–say, late middle school–my husband decided it was more fun to stay home with his dad and set up elaborate scares for the neighborhood kids than to go trick or treating himself.
One year, my FIL put a big amp out in the yard under a table with a cloth over it. In the middle of the table, they put a HUGE bowl of candy. He and my husband hid in the living room with a mic and watched out the windows for kids. The kids, of course, would come up and look around, see the porch light on, and be confused. (You gotta be kidding me! We can’t be this lucky!) Some of the more suspicious ones would peek around looking for a person, but wouldn’t find anyone. Then, finally, they’d sneak up to get a handful of candy, and they’d scare the pants off of them! The kids would laugh and think it was fake after they got over the initial “boo!” scare–but then my husband would start making comments on their costumes and they’d get freaked out.
After my husband went to college, his brother took up the cause with their dad. One year, my FIL sat on the steps handing out candy, and my BIL had dressed up in a really elaborate costume that showed only his face–and that was covered in makeup. He looked like a Victorian era wax sculpture–only incredibly lifelike. He can also stand eerily still. Even adults who brought kids up were fooled and asked my FIL if they could “touch it.” Of course, he’d say sure, go ahead! My BIL says he had the time of his life waiting for exactly the right moment and way to startle each person. If someone leaned in really close, he’d just turn his eyes toward them and flinch a little, and they’d jump back. He got a group of teenage girls really well by jumping off the steps suddenly–one shrieked and ran about 10 feet before stopping and turning around. 
This wasn’t Halloween, but happened during a science fiction convention in Kansas City. After the costume display a group of us, still in costume, went walking on the Plaza. We saw a car pull up at the corner to wait for us, with a couple of kid sized faces in the back. One guy, with an alien mask on, leaned down to look in the front window, and that’s when the real little kid in the front saw him. Started crying and screaming. The two older siblings in the back thought it was cool. Guy in mask felt awful and took it off to apologize to the parents, but they told him it wasn’t his fault.