I don't think children read comic books anymore, but 60 years ago I'd have understood it if it were described in terms of "alternate" universes, something like this:
- Earth-0: the original apostolic biblical Christianity
- Earth-1: the Roman Church
- Earth-2: the Eastern Church
- Earth-3: the Protestant Churches
- Earth-3.1: the Anglican Church
- Earth-3.2: the Presbyterian Church
- Earth-3.?: …
- Earth-4: the Mormon Church
- Earth-5: the Jehovah's Witnesses
Some event causes a divergence and splits off a separate world.
The people in each world have truths that would be considered false in many of the other worlds.
- One world knows that the Flash is really Jay Garrick, while another knows it is Barry Allen.
- One world knows that Mary was born without sin and is now in heaven listening to our prayers, while another knows that Mary was an ordinary woman who is now dead and buried.
The child will understand the concept of groups of people believing things that completely contradict what other groups believe, without having to feel that any of them are "wrong".
What each group believes can then be taught and studied objectively for what it is, not for whether it is "true".