How do I reset my child's apple id password when using Family Sharing?

For some reason, the password we have for my 9 YO isn't working. We'd like to reset it but can't quite figure out how to do that with a subordinate account in Family Sharing.

He can access the device, we have access to our devices, etc. This was triggered by a password challenge for buying a game, which is a new UX - previously it would send an approval request to one of our devices. Now it wants his password which is a bad UX if intentional...or maybe we tripped across a bug?

Anyway, the saved credentials I have for his account don't seem to work. How do I reset his password?