In the early days of the Star Wars franchise, fans were happy leaving a lot of information to the imagination. Today, every detail can be explored through the power of expanded universe fiction. Mandalorian culture is a great example. It used to be little more than background details, but as The Mandalorian goes deeper into the journey of Din Djarin, fans learn more about the unique culture.
The Mandalorian is primarily the journey of one grumpy reluctant dad and his superpowered alien son as they travel the universe. Along the way, the aforementioned dad frequently has to tangle with the strange rules of the obscure militaristic cult he claims membership to. He was recently kicked out for daring to take off his hat. The rules are clearly strange.
What is a Child of the Watch in The Mandalorian?
The Children of the Watch are the most conservative and traditional sect of Mandalorians. While most other surviving Mandalorians have long since abandoned the oldest rules, the Watch holds fast to them. Children of the Watch are required to keep their helmets on at all times, never revealing their faces to another living being. If they should ever disobey this rule, they'll be barred from wearing the helmet again. The specific group within the Watch that Din Djarin belongs to is referred to obliquely as The Tribe. Though Din Djarin is no longer a member of The Tribe or a Child of the Watch, it's still an important part of his story.
The Children of the Watch are comparable to the orthodoxy of any major religion. They seek to bring all of their culture back to the rules that once governed them. For his part, Din Djarin was completely unaware that there were any Mandalorians who did not live by their rules. This implies that foundlings simply aren't informed of the members of their culture that their elders would cast out. The Tribe seems to consist of roughly 40 members and an unknown number of foundlings. It's hard to know how many Mandalorians exist elsewhere after the Great Purge savagely reduced their numbers. The Tribe's strict religious beliefs make them unpopular, and though most citizens can't tell the difference, it's interesting to see an extremist group within the warrior culture. The Children of the Watch are extremists, but they have an interesting history that could explain their views.
Where did the Children of the Watch come from?
The Children of the Watch formed sometime before the fall of the Galactic Empire. As a splinter group of religious fundamentalists, they spent their lives cloistered on a distant moon. Since they weren't on the surface of Mandalore, they escaped the fiery destruction of the Purge. For years, they established a covert on Nevarro, where they could live in peace. While there, only one member of the Tribe was allowed to leave the cloister at a time. This policy ended when the entire Tribe banded together to save Din Djarin and Grogu. Though much of its history is shrouded in mystery, there are some hints to its past.
While this theory remains unconfirmed, many Star Wars fans believe that the Children of the Watch descended from an older Mandalorian group called Death Watch. After years of disastrous civil wars, a pacifist took over Mandalore for the first time. In response, a splinter group of dangerous terrorists formed to keep war at the center of Mandalorian culture. They weren't very successful, but they got to take over when the evil Darth Maul absorbed them into his hostile occupation of the planet. Death Watch was disbanded during Darth Maul's rule, but it's possible that the philosophy lived on in the form of the Children of the Watch.
Death Watch was founded by Pre Vizsla, a descendant of the original Mand'alor Tarre Vizsla. Paz Vizsla, a member of the same clan, is one of the preeminent Children of the Watch. Din Djarin, the Watch's former hero, wasn't born into Mandalorian culture. He was rescued and inducted as a foundling by a member of Death Watch. Interestingly, as Wookiepedia points out, the French version of The Mandalorian translates Children of the Watch as Les Héritiers de la Death Watch. Adding the missing word back in strongly implies that the Children of the Watch are direct ideological descendants of Death Watch.
The Children of the Watch are interesting in a variety of ways, but there's still a lot about them that's kept in the dark. Exposition about the minutiae of Mandalorian cultural schisms is the last thing anyone is watching the adventures of Din Djarin for, but hardcore fans might be excited to see the throughlines. The Children of the Watch might be a wholly new sect that appreciates the goals of Death Watch, or it could be its new iteration. Either way, the group that raised Din Djarin still holds many secrets.