Approving a purchase on child iPad

We got our 4 year old son an iPad for Xmas. We downloaded an educational app that requires a subscription. I want to approve the subscription, but the only apple device in the house is theiPad. I know to download apps, I can approve it directly on the iPad, but I don't see a way to approve the subscription anywhere.

The iPad is the only apple device in the house and it will remain the only apple device. Is there any way to approve the subscription from my android phone or windows computer or on the iPad itself? I tried to sign out of his child account so I could sign into mine to approve it, but the iPad is set up in such a way that he can't be signed out apparently. Did we waste money on apple garbage that we can't fully utilize without being an "apple family"? We've been happy with the thing so far, but I am going to be pretty disappointed if we spent all that money on an iPad and can't approve things without being pigeonholed into buying more apple products (which I refuse to do. I shouldn't have to buy another device to make the first one work properly)