The opinion of Rabbi Moshe Feinstein -- which I've heard quoted by several prominent American Ashkenazi authorities on Laws of Nida -- is that the "modesty" required of a woman in her own home, when she's a nida, is not the full set that would apply for going out in public (e.g. hair covering). Rather, it's whatever she would normally feel comfortable wearing inside when it's just her family around. (I presume when she's not a Nida, her kids don't see her walking around naked -- but very likely they might see her in pajamas.) Hence in virtually all cases, hair covering in the house wouldn't be required. (Yes if you want to be extra pious and holy and stuff ... but as Rabbi Feinstein warned elsewhere, don't be extra holy and pious at the expense of straining your marriage.)