This isn't really a belief, more of an experience.
When I was in third grade, I was convinced that 'bad words' did not exist. If someone told me that I'd said a bad word (and they were mistaken, because I never said anything worse than 'stupid'), I would ask them indignantly how a word could be bad. The same thing happened if I accidentally used my middle finger to point to something - it was just a finger!
One day, while there was a substitute, we were working on something about the states and a few kids in my group had just realized that Massachusets, when said with an odd accent, sounded like "Massachu-shit." They were laughing hysterically at this, and I thought that "shit" just sounded funny to them and had no meaning, so I said it once or twice to get a laugh.
A few minutes later, one of the kids had somehow snuck away (the very same one that taught my my new word) to tell the teacher that I had said a bad word. I, of course, had absolutely no idea I had, and I didn't say the word "shit" until the seventh grade out of sheer embarassment.
A childhood belief about swearing

others believed things like this
- My sister used to think that "spit" was a bad word. She meant "shit".— "Spit!"
- i used to think that the word "stupid" was a curse word, ranging in scale right beside the "f" word.Anon
- In first grade, I thought that if I've never heard of a word before, then it wasn't a word.— Destiny
- Up until 8th grade or so, I was convinced that I invented the word "Fart"— Jonah
- I used to think the word fever was just another word for giraff— Marshall m
- I used to believe everything starting with f was a bad word after I learned about the "f-word". — Sarah