what exactly is the problem? Could you describe the problem a bit? Just from the screenshot I am not 100% sure what you mean.
If you have problems to set up Stadia for your child, you can also check this article for guidance.
Please let us (the community) know if we can help you any further, we are all happy to help.
In the Stadia app on my phone, I click the link provided (you can see some of it in the first image). Then I am taken to the error page you see in the second image. As you can imagine, I have a browser on my phone (Chrome) and I assume the link should resolve to a page with some more information or some action I can take, but it fails.
Apart from pointing out this error, I am curious to know how Stadia family will work. I understand the control of purchases, but will family members share the games purchased on the parent account? Is there a page you can point me to with information such as this?