Many iPhone X users have discovered that they are unable to use Apple Face ID to approve family purchases, resulting in them having to enter their passwords manually whenever another user on their account tries to buy something. It's not a big deal, but you probably won't relish the thought of punching in your password every time your little ones want a new game for their iPads.
Basically Ask to Buy is an iOS feature that allows parents to control what their children are buying with their phone. It is useful, given that children have been known to rack up hundreds of dollars of purchases by mistake.
If you own an Apple iPhone X you still can't approve your kid's app requests. To approve the request the family organizer type in his/her complete Apple ID password to each and every request. Such passwords could be extremely long and tiring to type.
According to Verge, the lack of Face ID verification affects all transactions on iOS' Ask to Buy, the Apple system which allows parents to monitor what their children are buying online.
It's odd that Face ID suddenly fails to work for Ask to Buy, Apple introduced the new feature so that it will automatically replace Touch ID on many third-party apps. But threads on the official Apple forums show that some users are getting slightly frustrated having to re-enter their passwords several times in a row.
Since it was discovered by many that Apple's Face ID can be fooled by a family member who has a strong resemblance to an iPhone X owner, Apple might not want to take a single chance that a daughter who looks just like her mom, or a son who is the spitting image of his dad, hijacks a Face ID approval, and turns it into an unapproved shopping. At least some believe that this why Face ID cannot be used with the "Ask to Buy" or "Family Sharing" features.
For now, there is nothing from Apple that gives us a definitive explanation about the issue, we will keep you update if Apple makes any note on this particular issue.