There's nothing wrong with teaching children what you believe, even if it ends up accidentally maligning another person. Such as if I tell my girls to dress modestly, because God wants us to be modest. Children will draw conclusions from that, such as, "If someone isn't modest, they aren't doing what God wants them to do."
Fine with me.
If a child's parents believe that jewelry is a sin, then they should teach the child that belief. Not only is it their obligation, it's their right as parents. If the child then sees someone wearing jewelry, and thinks they are sinning, that's a natural conclusion--and children are blunt. LOL!!! It doesn't mean the parents are wrong for teaching the child what they believe.
My children have been taught that it's a sin to fornicate and shack up before you're married. So if they meet a couple who is living together, they immediately assume they aren't Christians.
Again--fine with me.
You're really making a mountain out of a molehill. Jeff was in the grocery store with Jeffrey one day, when Jeffrey was about 3. A man was checking out, covered with tattoos, even on his bald head. Jeffrey pointed, and loudly told his Daddy, "Daddy! There's a BAD MAN!!!!"
Who knows how he made that connection, but he did. It was awkward for Jeff at the time, but funny to all of us later. *shrug* Kids say silly things. It doesn't mean their parents are teaching them to be judgmental or mean.