https://arstechnica.com/gadgets/2017/12/parents-cant-use-the-iphone-xs-face-id-to-approve-family-purchases/

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.