Contrary to popular belief, there's no evidence that drinking coffee will stunt children's growth (although there may be some good reasons for them to drink only small amounts of it, if at all), a belief that was popularized by an unscientific 19th-century advertising campaign for a grain-based hot drink called Postum:
It’s a myth: There’s no evidence coffee stunts kids’ growth - Salon.com
It’s a myth: There’s no evidence coffee stunts kids’ growth - Salon.com