For most of my childhood I thought that everyone in Canada spoke French (my only experiences were in Quebec).
My little one has already experienced this phenomenon and she is just 8.
This was when she was 4 or 5. We were driving by the local Volunteers of America store (like Salvation Army) where she loved to dig through stuff and she asked if we could stop. I explained to her that, it being Sunday, the store was closed but she didn’t settle for that and wanted to know why they weren’t open when “we” wanted to stop. So I explained that the people who worked there wanted to be with their families and whatnot and couldn’t work all the time, adding, “They don’t live in the store you know,” meaning it sarcastically.
Well it turns out she thought they did. So we talked and I explained things. When we were done she got very stern and said, “Well, you got to TELL me these things. I’m just a little kid you know!” She was generally upset with me that I had overlooked this part of her education.
We do. A lot of us, though, speak it so badly that it sounds exactly like English. 
I must have been around twelve when I realised that the excruciatingly-long-and-boring wait in a restaurant between when you ordered your food and when it came was because they had to cook it! I thought that all restaurants were like burger places and cafeterias and the food was pre-made and kept ready.
Whenever I was out with my Dad buying stuff, like at a hardware store, etc. he would pull out his wallet, take some bills out and say “Here, that ought to cover it”.
So later I got to thinking that what something cost was how much money it took to literally “cover it”.
I further thought this must be true because to buy a car or a house was very expensive because of the amount of bills it would take to “cover it”.
Speaking of such, I thought when they went over your house with a fine-toothed comb that they literally took a black haircomb and combed through the dust and dirt on your floor and in your rug, looking for tiny clues.
I was constantly going to the hospital for doctor’s visits from the time I was born until I was about 2 years (and at decreasing intervals after that into high school) because of some congenital birth defects involving my hands. I usually had to visit several areas during the appointments.
Every time we had to go to other floors in the building, I thought that, when the elevator doors closed, we stayed still and the floor we needed somehow moved so that it was outside the elevator door when it opened up again.
Also, for years, even after I heard it pronounced correctly and in context, I swore “recipe” was pronouced with two syllables and a long “i”.
And having decided Puerto Rico was pronounced “Puerto Rico Ryso” (yes, I added a word, a couple of syllables – it was all those “o’s”) I was loathe to change it for many years.
I think the word thing was because I started reading so early and didn’t ask or look up how a word was pronounced.
Well, according to general relativity, you’re right!
Not so. From inside the elevator you are able to conduct an experiment which demonstrates that you, not the building outside, is experiencing acceleration. 
At the age of five or so, and having read the Ladybird book “The Fireman”, I thought it would be a good idea if firemen took their fire engine and drove it up to 60mph and then turned the engine off… so that when they restarted it, it would already be doing 60mph and they would get to the fire that much sooner.
At an even tenderer age, through mispronouncing the word “clothes” as “cloes”, I thought there was a singular form “clo”.
You’re not unique in this! All my children have said “clo” to refer to one article of clothing. 
You should have gotten Chinese counterparts to start working on their end; then the effort needed would have been halved.
When I was about 6 I thought the word ‘corduroy’ was McQuarteroy.
I thought if you turned off the TV in the middle of a show you could go back later and finish watching it.
I thought commercials were just helpful public service announcements–actually maybe that wasn’t so outlandish in view of the fact that even then there was a great deal of toy product placement in children’s TV shows.
I thought the Beatles were singing, “She loves you – yay, yay, yay”, like “right on!”.
I could go on…
With superhero comics and related things, I had a very hard time with the occasional issue or episode where the superhero “goes bad”.
I had a book about deep sea divers that had a lot of pictures, some of divers wearing those weighted shoes for working on the ocean floor, and some wearing flippers. I thought if they stayed under long enough the shoes would gradally transform into flippers.
I thought Las Vegas was a state, and that the West Coast city just north of the Mexican American border was called “Sandy Ego”. This was before the waffles existed. And the “sandy” made sense; after all there were certainly beaches there.
ME TOO! That’s the most frightening book on the face of the earth. I was terrified of everything for weeks after reading it. WEEKS. My mom had trouble getting me to leave the house. And I wouldn’t read Dr. Seuss again for about a year. Oh, my God, the horrors. I’ll never pick it up again, even though I’m now well into adulthood.
I used to bury popcorn kernels in my backyard, thinking they’d grow to become some sort of popcorn tree.
I used to think there were white Asians and black Asians, just like there were white Americans and black Americans. It used to bother me that I never came across any pictures of Chinese people with dark skin.
Like Guin, I thought sex was just kissing with no clothes on. That’s what movie love scenes looked like to me. Just people rolling around in bed naked, in a lip lock. I figured out that babies were conceived through some transfer process, but I thought the mouth was the conduit for this transfer.
Yeah, hold you breath? Somewhere in the world they’re a picture of 6 year old me tap dancing on a grave. Well it was this massive three foot high, six foot slab of marble. Just called out for a Shirley Temple. My family thought it was cute. We used to take walks through the graveyard after dinner. Very peaceful.
(More recently, I took pictures of my boyfriend naked in a graveyard (him naked not me). Something about the way of all flesh…or something.
One of the things I always believed when I was a kid was that fire alarms with the little glass vial that gets broken carries sound through it all the time, but when it is broke the sound can come out. Some friend told me about it and I believed every word of it. Looking back, I’m gullible.
Brendon
When I was younger I thought that time was not linear, but repeated itself. So there would eventually be another me, there was another Abraham Lincoln, etc., and none of us could know which us we were because time simply repeated itself.
I firmly believed this until I was about 12 years old. I have no idea where I got the concept from.
~Tasha
We had a set of Time/Life science books that I liked to look at the pictures in before I could read.
One was on the solar system and showed planets/galaxies etc.
One was on unique geographical features of the earth and showed places like volcanoes, ocean floors, deserts, canyons.
Another was on unique animals (living and dead) and showed giant squid, mammoths, dinosaurs, whales, platypus.
For whatever reason I had it in my mind for many years that book one showed planets in the galaxy and the other books showed the terrain on these planets and the animals that lived there.
When I was in elementary school, I was a nerd. I wanted to know how the fire alarms worked. My “intelligent” friend told me that the little glass vials contained a constant siren, and when you pulled the thing down and broke the glass, it “let the siren out” so people knew there was a fire. I completely believed this for at least a year. Finally, I figured out how it worked, and that it had nothing to do with a little vial full of sound.
Brendon
Likewise, when I learned all 50 states as soon as I could read, I naturally assumed that since Arizona was desert, the name was derived from “Arid zone.” What could be more obvious?
When I was three or four, I somehow got the story of the Resurection of of Christ confused and thought that meant that EVERYONE would eventually come back from being dead. It didn’t scare me-on the contrary, I thought it would be so great to be able to meet my great-grandparents.