I think that I spat in a kid's ear
Why would you spit in a kid's ear?
I had some idea of how to do it.
There's nothing wrong with a harmless prank here and there, but doing something gross like that is pushing it a bit too far. You should apologize for doing such a disturbing thing to them.
Nothing happened so stop being offended by it.
The kid may have been disturbed by it though. I still think you should apologize.