New iOS 11.3 feature allows parent's family purchase approval with Face ID
In iOS 11.3, the ability to approve family purchases with Face ID will be included, possibly by popular demand, as until now you always had to enter your long password to grant purchase authorization, as the iPhone X has no stinking Touch ID to speak of which can be used on lesser iPhones without a problem.
Granted, you still have to provide your password the first time your kid wants to grab something off iTunes, but now the iPhone X will ask you if you want to enable Face ID for subsequent purchases, and you can say yes, saving yourself the thumb gymnastics the next time your kid starts buying games with your face while you are sleeping on the couch.
source: 9to5Mac
Things that are NOT allowed: