sanity through user accounts
Many households have one computer for the family. Some clients I’ve come across are all using the same computer with the same account. While this can be done, it can creates all sorts of problems with missing or accidentally deleted files with no privacy on the computer between family members.
The simplest thing to do for a shared family computer is to create separate accounts for each member of the family. System Preferences:Users and Groups: is where the Administrator of the computer can create new User Accounts. This way Mail, Calendar, Notes and many other Applications can be configured for whomever is logged in. The child (or spouse) does not need to have an Apple ID to have they own account on the computer, though many Apple services require one.
MacOS allows for Users to switch access to the computer without disturbing each others work through the Fast User Switching menu. 
Many people, many devices.
Some households include a Digital Device for each member of the family (even pets are getting trackers). Managing all this is part of Apple’s Family Sharing and Screen Time offerings. These allows the Family Organizer to manage payments for, and access to, Applications and Devices, as well as locate members or even lost iPhones or iPads.