Nanna's Knits - if you're saying "various knitted things that have been made by Nanna".
Nanna Knits - if you're saying "Here's Nanna! She knits."
The apostrophe should never be with the Knits (no Knit's ever), unless you're saying that her name is Nanna Knit and something belongs to her. i.e "Nanna Knit's lovely socks" would be right if her name was Nanna Knit. But if you wanted to say that she made lovely socks it should be "Nanna knits lovely socks".