Umbran has it right. The best way to teach nearly every game to children is by starting it easy and gradually ramping up difficulty. Children learn fast, but you need to let them grasp the basic rules of play before you make the game challenging.
There is also another consideration nobody here seems to take into account. The game is simple and it does not allow for a lot of tactics. You don't have any good methods to protect someone from harm if monsters decide to focus attacks. That's an important, though metagame, reason for them not to do that. Monster tactics is unrealistic, but you can't get realistic result from a mechanics appropriate for children 6 years old.
There is also another consideration nobody here seems to take into account. The game is simple and it does not allow for a lot of tactics. You don't have any good methods to protect someone from harm if monsters decide to focus attacks. That's an important, though metagame, reason for them not to do that. Monster tactics is unrealistic, but you can't get realistic result from a mechanics appropriate for children 6 years old.