iPhone X owners have found that Face ID isn’t available as an authentication method for the “Ask to Buy” feature, which allows parents to approve their kids’ iOS purchases and downloads. Instead, the parent (or any other “family organizer,” as Apple terms it) must enter their entire Apple account password to approve each individual purchase attempt. Users are frustrated because equivalent functionality was available on Touch ID devices, and that functionality has been lost in the transition to the iPhone X. Face ID can be used as an authentication method for other purchases, just like Touch ID before it — but Touch ID also worked for “Ask to Buy,” and Face ID doesn’t. […] Parents of large families with several children, each of whom might have an iOS device available to them, will find that the requests mount up quite quickly — especially right the holidays. Kids cashing in App Store gift cards add to the requests already coming in from normal use and in-app purchases in games.I only have one kid, but I noticed this too in the post-holiday gift card bonanza. I can’t really think of a good explanation for why Touch ID can authorize these transactions but Face ID can’t.
Samuel Axon, writing for Ars Technica: