I think any child should be aquainted with all the major religions and be generally familiar by adulthood with their texts, major figures and historical context.
Teaching it can be as simple as having copies of the texts at hand, watching documentaries and discussing.
I happen to BE raising kids in the modern era in just this way. I am not religious (but do hold my own spiritual views) and being aware of the religions of others is just a part of life. My mother and my sister-in-law and a few of their cousins are Jehovah’s Witnesses…naturally, the topic of what they believe comes up now and then.
We have Ethiopian, Muslim neighbors. Ditto.
Other neighbors are Catholic.
etc…
As I said, I am not religious and have never pushed any such beliefs on my kids.
In fact, I really pissed my mother in law off by refusing to allow her to take my then 4 yr old daughter to her Southern Baptist church…uh, HELL NO! My policy is that I will protect them from such exposures, esp. when I am not present, until they are older, say 9 or 10 at least. At older ages, they are free to go or not.
But to turn them over to someone who has the intent to convert them at such an age? (as she did) No way.
A well rounded knowledge of the world’s religions is as vital as a well rounded knowledge of political systems, history and science (and it is virtually impossible to fully understand any of those subjects without referencing religion, at least).
So like Frank Zappa, I don’t teach them anything except as history or myth and let them grow up and make their own decisions. I can only hope they will be well informed ones.