OK, when I first saw this thread, I thought it had to do with dressing up for Halloween. Like do you put your mask on over your hijab... or what.
I've personally never encountered any metal niqabs...and to be honest, I've seen very few Niqabi sisters too... both in a very Muslim area of Brooklyn and in other places I've lived.
If you're afraid your DC might be scared, you might want to see if your library has the book "Muslim Child" by Rukhsana Kahn. In it, there's a story called "The Black Ghost" about a boy's Mom who wears niqab. "The children run from fear of her and her young son is dreadfully embarrassed until the black ghost rescues one of the boys. Reaching out from under her black abaya, the mysterious woman is soft and gentle. The young boys confront their friend, "You never told us you had such a nice mother."
It's really a nice collection of stories for kids and even includes a recipe for Samosas.
Fwiw, I have never seen the metal face covering that has been mentioned...a black cloth nicab yes.....It looks cultural to me more than anything. I am going to ask a good friend of mine if she knows anything about this kind of covering.