The trip is to a mosque and the girls (year 3 and 4) have to cover their heads with a scarf, the boys don't have to.
Just to be clear about where I am coming from, this is about a specific practice which I believe is discriminatory and therefore disagree with. I would not choose myself to enter a building that I could only enter if I wore particular clothes but where that requirement was limited to one gender or one group defined by arbitrary characteristics. I would be happy to cover my head as a sign of respect/tradition if everyone entering the building was required to do so.
They can make their own decision on this when they are adults.
Am I being ridiculous and petty or is it reasonable to stand up for my view that just because a custom is part of a religion that does not excuse it being discriminatory?