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.
Apple has touted Face ID's ability to replace Touch ID in interactions for which Touch ID was previously used. In fact, we found when reviewing the iPhone X that third-party apps using Touch ID automatically used Face ID instead on that device, with no action needed from Apple. It was a slick, seamless transition, so it's all the more surprising that it doesn't work for an iOS feature offered by Apple itself. Apple's documentation of the feature makes no mention of either Touch ID or Face ID.