Additional user blogs not taking on the theme of the main blog

I'm using the Product theme (child, with a custom background) which is working fine on the main site.
I've created a test blog, and it's not taking on the theme of the main site, but reverting to the Kubrick theme.

I've read this:
And I've followed the instructions.

Under Site Admin>Themes, I have the Product Child active only. No others active.
Under Blogs, I go to my test blog, click Edit, and on the right I can see all themes that are not activated. I can see the Product theme, but no option to tick the Product Child theme.

I tried various combinations, but I can't get the Product child to work on additional blogs.

How can I change this?
How can I ensure that additional blogs take on the Product child theme?

Thank you.