Not only as a child.... my missus (70 now) used to believe it was impossible for horses to run with all 4 feet off the ground at the same time... "its not possible, they'll fall over..." i then got her to watch some horse racing and athletes back in slow-mo. Voila... case closed
Same on the horse comment (despite movie posters and such showing stills with horses in full stride with all feet off of the ground). Horses weren't the only thing, though - we were told by some adults that the same rule applied to people. It's fairly hard as a young kid to confirm that you're continuously running with feet off the ground for a large part of the time .....why? What happens if you run and stare at your feet? At some point, you'll trip on something or run into something.
This occurred at the same time as the sport of speed walking was becoming popular. "why do they need a rule that they have to have feet on the ground? It's impossible to have both feet in the air while walking or running".
That sport died out pretty quickly here - in less than a decade (walking as fast as you possibly can while intentionally having one foot on the ground at all times to avoid disqualification - looks painful. The faster someone goes, the dumber it looks, and there was at least one time that I can recall seeing a televised speed walking event with prize money. it's almost worth finding a video - it made people look like awkward ducks and when you laughed at people at our local rec park or pointed, they got really pissed).