I have no idea if feminists worked on that particular ban in Germany or not, though I suspect some did. You might be correct in assuming that the rights of the girls themselves did not play a big role in what actually created the ban, though as I said I don't know. But in general groups not caring about women's rights often use women's rights or women's dignity (often opposites) as a pawn in their games. This is true of the US under Bush, of many right-wingers, of the Salafis, of ISIS and so on.

I think the reasons for bans such as this one are probably different for different groups supporting them, ranging from the practical arguments that wearing, say, a burqa, will seriously hamper various normal activities that children should be able to participate in, such as play in the yard (I once observed what this does to a child when one girl stood alone in her long abaya while others, dressed in shorts and t-shirts, played in the climbing frames), to, as you state it, western prejudice of Islam, though these causes can interact in complicated ways if the way girls are supposed to play in the first place is religion-dependent.