There is a beautiful old Catholic cathedral in Denver (Basilica of the Immaculate Conception) - I’d suggest they visit that, too (wouldn’t have to cover their ankles or head).
http://www.denvercathedral.org/
I’m glad my nephew goes to a Catholic school (has been there since Kindergarten). It’s a very good school with great kids and parents. His parents plan on him doing so through middle school and high school. My sister refuses to put him in a public school and I totally support that!
however, they would have to cover their arms. I was unable to visit a cathedral in Florence Italy as I had a sleeveless top on (not tank or tube, just sleeveless). I could have “rented” a scarf for a ridiculous amount of money for the quick trip inside or I was barred from entry. I stayed outside. So don’t tell me cathedrals dont have a dress code.
I don’t agree with the visit to the mosque, but if you go inside their building, you follow their rules. The problem lies with being forced by a public school to go inside their building. My kids would stay home as well. They are well acquainted with the inside of the only religious buildings (churches) that I want them to visit.