The first dog I took to my house, my kids asked if they could keep him so I said yes and now he lives in my house. I think that was Bran. It was one of the Dawnguard dogs, anyway.
I now have Sceolang (the other Dawnguard dog) following me, but my kids never comment on him. Does that mean I can't keep Sceolang in my home as well, or do I just have to wait a certain length of time before they'll ask about adopting him?