Of a specific cult/group/religion of mandalorians, 'The Children of the Watch'. It is brought up in season 2 by Bo Katan that not all mandalorians follow those practices.

But the other ones also say it out of respect? I’m surprised how much I am enjoying the show, but I wish they hadn’t showed us his face in season 1.

Maybe, the children of the watch are a subgroup of 'Death Watch' if I have my lore right (primarily concerned with 'foundlings' or adopted war orphans). Deathwatch was a much bigger deal around the end of the clone war. I think 'Clone Wars' covers them pretty well, however I've never fished through all of it to find the episodes to complete the storyline. At one point they pledge themselves to Darth Maul and take over Mandalore for a time.

The Mandalorians that Din Djarin primarily concerns himself with in season 1 are all children of the watch. I'm not sure that you ever hear it uttered by Bo-Katan or her friends or Boba Fett, who are not children. However Bo-Katan was part of Death Watch before she broke away with her Nite Owls, so if it was a Death Watch thing she may have uttered in during season 2.. I don't recall.