How to add Super Duolingo for a child

One of my kids has taken it upon themselves to use Duolingo to learn Welsh. This morning they were upset to find that they’d just lost a 6 month+ streak. They could “repair” it with a free trial of Super Duolingo, but when they tap the link for that it requires them to choose a payment plan (to kick in automatically at the end of the trial if we don’t cancel), and that of course requires an adult.

The thing is, they get a screen that says “You need a parent to do this,” but when you click OK it just dismisses that screen, without asking for my Google account or password or offering me any means to authorise it for them. Duolingo Help is just a list of FAQs (and evidently this Q is not A’d very F) and Google Family Link Help is another maze of blind alleys.

Does anyone have any idea how I can authorise a free trial of Super Duolingo for a child?

We’re both on Android / Duolingo app / Google Family Link / 12yo child / I have Super Duolingo (for the Gàidhlig, not the Welsh!) individual paid.