Simple way to scare them: Buy a knife, and if you bump into them again; whip it out (the knife I mean), and say something along the lines of "you wanna fuck with ME? come on then shit head, lets dance" all the while, have a creepy, grinning face on, cackle and thrust at them (but don't hit them), even if it doesn't scare them (and it should), it will convince them that your fucking crazy, and not a person to fuck around with.
On another note: How old are these kids? scaring eight year olds might not be the best idea.....