Our Mosque was in East London, pretty Muslim area and certainly no separate entrance for the school trip, and I don't think young girls (they were Y1) needed to head cover, although adult helpers did. Not really sure what the issue with head covering would be anyway. Lots of temples etc. when you are on holiday require arms to have some covering and won't accept shorts etc. People do it without question as it they want to go and see a historical site, but need to follow that site's rules. What's the biggie? I do hope some of these kids when they grow up will go on holiday and do these kind of things and not think, "OOOh my Mum was funny about me going in a Mosque where I needed to cover my head, so I'm not going to visit this rather impressive and beautiful temple."
You don't mention whether the adult helpers asked to cover their heads were male or female. Nor do you mention a specific sex in the rest of your post. You say 'kids' when actually you mean females, don't you?
I would never subject my daughter to sexist treatment disguised as 'religion'. I would not allow her to cover her head 'in modesty' (ffs) when entering a building when the boys in her class do not have to.
Even if she was not asked to do this due to her being a child, I would not be happy for her to witness her teacher having to cover her head purely because she was female.
Either everyone has to do it, regardless of sex, or nobody does.
Tradition, culture, religion - all crappy excuses for treating females as an inferior sex.
I'd actually be quite proud if, when my daughter grows up, she chooses not to visit a place that holds such disgustingly sexist entry requirements. There are plenty of impressive and beautiful holiday destinations she can chose to venture to that will not treat her differently because of what's between her legs.