Science fiction has some element of factual science involved, whereas fantasy does not have to involve any science whatsoever. Science fiction often takes something that is technological, medical, natural, or scientific in some way and expands upon it in a way that has not been discovered or explored yet. It may be plausible or possible but currently is not in existence.
This genre appeals to children because it combines reality with things that come from an author's imagination. Deep space travel where people colonize new planets only to be chased by alien beings that may or may not exist in the real world. Scientific machines and algorithms that allow investigators to travel back in time to chase a bad guy who is trying to rewrite history. These are fantastical ideas that seem completely possible for children.
Fantasy Fiction
Fantasy fiction is an imagined or dreamed tale where unrealistic events and creatures exist in conjunction with a fanciful tale of adventure or magic. The majority of the elements are not and cannot be real. They are purely works of the author’s imagination such as a golden ring that turns a hobbit invisible, or a sword that only works to destroy evil.
Fantasy fiction appeals to children because there are no rules. Typically, there is a hero of some kind that a child can relate to and he or she will rise to great heights both metaphorically and physically to battle evil and save the land. These may be physical feats of strength and bravery or intellectual puzzles of whit and intelligence. Fantasy fiction is an entirely different world in each and every book.