The best thing I've heard children called is "teacup Humans". (True Blood)
Though new is the opposite as old when it comes to a material object, young is the opposite of old when referring to a person.
Children are called young people, little ones, youth, theres really a lot that children are called.
Because it sounds ridiculous and they're not "new", newborn are "new people" children arent.
Because most people don`t think of them as " new " people , they call them younger people.
