Certainly everyone has some embarrassing things they thought were true as a kid. Me, I thought that chipmunks were baby squirrels. A recent 2ch forum thread (later rounded up and commented on at Itashin!) took up this at times awkward topic and we selected 10 particularly cute ones. Note that our highlights are mostly skewed toward things that seemed unique or particularly Japanese (e.g. not Santa, although he came up). Please remember that since it was just a forum conversation, these are not necessarily widespread beliefs.
When I was little I thought (paraphrased from Japanese)
. . . that there was a city in the United States called Downtown.
. . . that when you got married kids would just show up from somewhere.
. . . that all TV was live and how amazing it was that the actors in the commercials could do exactly the same thing every time.
. . . that there was only one ambulance in the world (and how amazing that was).
. . . that during Obon our ancestors came back as bugs.
. . . that if I told a lie I would be forced to eat 1,000 needles.
. . . that the bamboo shoots in ramen were pull-apart chopsticks soaked in soy sauce.
. . . that you went from: kindergarten —> elementary school —> middle school —> high school —> college —> Tokyo University (I thought Tokyo University was where you went after college).
How about you? What sorts of things did you believe when you were a kid?