I found 9x9 more discouraging, because if you haven't 100% mastered the tactics, you will brutally lose every single game. But on 13x13, you can use some strategy to gain something at least - you can abandon one battle and put two stones to their one stone in another fight, and feel like you've actually got somewhere. It might not be as good for actually teaching you tactics, but for a beginner I think it's more forgiving and therefore more fun.

Honestly it wasn't as hard as I'd expected. She's finished year 2 at primary school, so she's got the formal education framework with times tables etc, and she's been playing Minecraft (on creative) for a good while too, so abstract frameworks and rules are pretty natural now. So just saying "you take turns placing one piece at a time, and the area you surround is your points, the wall counts for surrounding, and if you surround the other players' pieces, you capture them" actually worked. I did give a few examples but she caught it very quickly. Edit: I think I also introduced liberties by saying that the pieces need at least one space to "breathe"

I found a travel set at a second hand shop, so when I said "hey do you want to try this new game with me?" she was happy to give it a go. This is the second session playing it

Hadn't heard of it! Honestly I'm a Go beginner myself. I've looked it up and I might give it a try!

Looks like it - it's called "The Igo Game", and all the instructions and most of the box text was Japanese. If you want to find it in your country, it looks like this:

https://www.animate.shop/products/4977513057677

and there's apparently a 19x19 version:

https://www.ebay.com/itm/394871612338

Removing stones when you capture them gets a bit fiddly with the small size, but I got it unopened for like $3 (Canadian) at Value Village so I'm not complaining.