In first grade, I was told by a friend that milkweed (you know, the stuff monarch butterflies eat or whatever) was lethally poisonous on contact.
Not something I was convinced of, but when I was in grade school I knew a guy who thought you could get to the moon by stacking dump trucks on top of each other and making all of them lift their scoops simultaneously.
When I was a kid I never made the connection between trials and getting arrested, despite being familiar with lawyers, courtrooms, and phrases like "tell it to the judge"; as a result I thought you got sentenced and jailed by the police if you were arrested, with no chance to argue your innocence. (On that note, until a couple days ago, my sister apparently thought you could be acquitted for any crime, including murder, by paying bail.)
My sister once believed that it was illegal to insult or make fun of the President of the US. Similarly, when I was about five, I thought that free speech was heavily suppressed in Britain, because I was always hearing about how great the free speech in the US is compared to some other countries, but Britain was the only other country I could think of. (I now realize that the intended comparison was to China, North Korea, and other such nations.)
Which reminds me: when I was... maybe five (?), I drew a world map from 'memory'. As I remember it, it was a big, completely empty ocean in the middle and a ring of land around it, making a sort of squiggly border near the edge of the paper. On the left side was the Americas, and Canada was at the top-left. As you moved right from Canada, you passed through "where the lions live". On the rightmost edge of the paper was China. The entire bottom edge was Antarctica. I think I added Australia in the middle of the ocean as an afterthought. I'm not sure how I forgot Europe entirely, or in that case where I thought Britain was.
When I was really little, I thought that when a man and a woman got married, the wife's body somehow knew "OK, baby time now" and then bam, a baby.