that I never gave a thought to the fact that Mrs. Norris didn't have children of her own. When reading the book it was just a given for me.

I believe that JA never mentions directly anything about the reasons the Norris's never had children or whether it distressed them.
But it is a very important point since I believe this history of Mrs. Norris molds her character very much. I was thinking, whatever the reasons for being childless, this could be the reason she wanted to take in a child from their youngest sister in the first place. She was the one who initiated the whole plan, and then when it was soon to be realized, she panics and tells Sir Thomas that Fanny should be staying with them. Possibly, having no children of her own, she wouldn't have known what to do with Fanny. And then there wouln't have been any other siblings in the house, etc. Mr. and Mrs. Norris would have been with Fanny all alone. I can imagine it would have been pretty daunting for her.