Ok, first you need one of those knives that looks real but the blade goes into the handle, these are like 5 bucks at any joke shop. You also need one or two friends to do this.
What you do is have one of your friends walking around with a big bag of candy or whatever and make sure he\’s near one of those groups of pre-schoolers and their parents.
Then, you and another friend just run up out of nowhere and start stabbing him with the knives and laughing, your friend with the candy should know to scream and fall over (it will be dark so people won\’t be able to notice the absence of blood and wounds, you can use anything wet to make it look like blood if you want to make it realistic).
Then you and your friend just grab his candy and sprint off. This will scare the shit out of the kids and their parents, I did it once. Just a hint, if anyone takes out a cell phone, make sure the kid with the candy runs away. I had someone calling 911 with me and my friends. This is pretty #%!@ing funny if you do it right.