Sharing here as this was a world of pain, and maybe I can save someone else this pain.
Information is as of November 2022 - earlier advice on the web is wrong, and this may well be wrong in the future too.
So, a friend gave us some cash as a present for our son and said "let him spend it in Minecraft" - sounds simple enough, right? He is a huge Minecraft fan, plays it a lot on his tablet.
The official way is to open up Minecraft, click "add coins" and pay with the device's payment method. We have a Google Family setup, which means in theory he can make payments but only with parental approval, and then payments can come out of the family account.
However - at least for our particular set of accounts, this just doesn't work. Payment doesn't work - even if I totally disable parental controls and say "he can make any payments", once you navigate through to payment you get a message "to complete this transaction secure your account" - with no indication of how to do this. 2FA? Something else? Nobody else seems to have an answer to this out there either.
So, I did some googling and found people saying "Add money to the child's Microsoft account and then buy the minecoins online" - that makes perfect sense, right? Nope. Maybe this worked once? I put £10 in his account fine - but there is no way, as of this date, to actually buy minecoins from this money. There is helpful advice from various help forums, and none of it works. I dug far enough to find a single "buy 500 minecoins here" page, which looked promising - but even there, if you click the "buy" button, you get an error "please contact Microsoft support". Yay.
(I wonder if there is any way to get that £10 back? I guess one day when he's old enough for a computer, he can spend it on something else)
Anyway, this used up almost an hour of fiddling, and with a complaining grumpy child saying "does it work yet?" (sigh - tactful polite behaviour isn't a thing at 5 3/4) and I gave up.
This morning I had the realisation - there isn't a link between the Android user and the Minecraft/Microsoft user. So I can bypass the whole parental control stuff by logging in to Android as me, and Minecraft as my son!
I tried it, and there is one wrinkle - Minecraft won't let you switch user easily! If I open Minecraft, choose "log out" then "log in" - it opens a browser window which remembers my previous login, and has no way to change it!
I did get past this - open a phone browser to https://xboxlive.com, log in, then choose "log out" and it deletes whichever cookie they use to track me.
Then I could log in to Minecraft as my son, choose "add coins", and payment came out of my Google Pay account. Finally. And when I open Minecraft on my son's tablet, I can see his precious minecoins there and spendable. I bet he spends them on something he immediately regrets…
So, the TL;DR is - if you want minecoins for your child:
- Log in to an Android device as an adult
- Log in to Minecraft as the child (see above for clearing your old login)
- Buy coins!