I have read to page 10, and learned a lot; no time to read further so apologies if what I say has been said before.
When I was a child it was common practice for females to cover their head in church and males to be bare-headed, out of 'respect for the Lord' (where that phrase came from I have no idea). Men always removed their hats indoors. Therefore the fact that boys keep their heads uncovered in the Mosque seems acceptable to me, cultural influences I suppose.
When I visited a Hindu Temple I was asked politely to remove my shoes; when I visited a Catholic church in Italy I was ordered, rudely, by a young priest to cover my bare arms before I was in the building;(ankle length dress, high neck and stole at the ready).
What I find most disturbing about this post is the message from the school:
Mosque visit: Please can boys wear trousers. Girls either tights or trousers plus head covering eg. scarf. Thank you
To me this appears as an attempt by the mosque to impose their beliefs about dress on non-believers, in the way they are doing in schools with relation to PE and general school uniform for their girls. Is the headscarf to cover the head in the presence of the Lord, or to cover the hair from the sight of the males? The dress code imposed on muslim girls, an interpretation of modesty, seems to say more about their effect on muslim males than about their religious beliefs.
When I taught in schools with a high proportion of muslim children I had to deal with male relatives who wanted the girls to wear shell suits for PE, including large apparatus, because 'the shape of the female form should not be visible'. T shirts, leggings, shorts, were not acceptable, and these were nine year old girls. Later some schools (near Birmingham) was forced to have separate PE and swimming lessons for girls and boys, with huge disruption to the timetable.
With regard to the original post, I think if they were my children I would send them on the trip, but question the uniform requirements; will the children be excluded if they don't cover their body in the way stipulated? It does sound as though this has come from the mosque, not the school. Tolerance should extend both ways.