Well, as has been pointed out: no theological reason for pre-pubescent girls to cover up. And in fact it is really rare to practically unheard of for this to happen in many Middle Eastern countries.
That having been said: if you are trying to argue that parents should not be able to determine which clothes their DC wear, you might find there's a whole rat tail of potential issues:
What about, for example, the synthetic fibre crap that catches fire if exposed to a flame half a mile away? Classical H&S issue! Should parents be able to dress their kids in this?
If so: what's the difference? Restrictive clothing doesn't kill - third degree burns do.
I'm not arguing in favour of religion, by the way, or in favour of being even more strict about it than theologically necessary. As an atheist I'm altogether in favour of getting rid of all religious beliefs.
The thing is, though: if you wish to restrict parents' ability to chose clothing for their DCs, you either have to be consistent or else it's simply discriminatory. If you don't: why complain?
Of course we could introduce a state impised uniform for all under 18 that would be mandatory. Might solve a great deal of other issues. I have a feeling it wouldn't be a popular idea for a whole variety of reasons, though.