Ever since the iPhone X arrived, Face ID has only ever supported one person -- unlike Touch ID, there was no way to register someone else you trust with your device. That won't be a problem with iOS 12. While Apple didn't discuss the feature during its WWDC keynote, iOS 12 lets you add an "alternative appearance" for Face ID. It's ostensibly meant for people who have problems with Face ID (say, people with glasses that significantly alter their look), but it can be used for a second person. A partner or child could have access to your phone without having to type in a passcode.
As with Touch ID, this isn't the same as true multi-user support. You'll have to reset Face ID for both people if there are ever any hiccups. It's still a big improvement, though, and it suggests that Apple is well aware of its technology's limitations.