Ask to Buy
When your kids want to buy an app, they have to tap an Ask to Buy button on the App Store. This sends a request to one of the parents, where they can approve it or not. Although Touch ID can be used to approve these purchases, Face ID can’t be used.
Although Face ID has a one in a million chance of getting fooled, that doesn’t apply to close family members, whose facial features are more similar. The probability of an incorrect match is higher for twins, siblings, and children under the age of 13.
Apple probably doesn’t allow Face ID with Ask to Buy as a precaution. Although, there should be a shorter passcode option, instead of having to type out a long Apple ID password, which some iPhone customers have been using since Face ID is so seamless.