Xbox game pass ultimate for child account?

I would like to buy the Xbox Gampass Ultimate for my son, since the offer is currently 3 months for 1 euro. When we click on join on his PC, the following message comes up:

Now to my question: how can I work around the problem?

It's best to ask your parents

As I wrote earlier, I'm the father. And I don't know what to do with the subject. That's why I turned to this forum.

You have to make a second account on which you are over 18 and with which you then confirm the purchase.