In Contacts, do you all have separate contacts cards? Is your son's information listed on your wife's card. This seems to be happening to others using IOS 12. You might want to consider getting you son his own Apple ID.
There may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos and/or in Photos/Preferences/iCloud. Then make sure full resolution photos are on the device/computer. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. Also set up Family Sharing to keep access to your apps. Also see Winston Churchill's response in the below discussion
Apple ID – Create
Family Sharing and Apple ID for your child
How to transfer all files from one iCloud account to another
Apple ID email address or password - What to do after you change
Family Sharing
Family Sharing - Use your own Apple ID for Family Sharing
Family Sharing -What types of content can I share?
Family Sharing - If you don‘t see your family‘s shared content
Family Sharing - Ask to Buy
Family Sharing - Ask to Buy - Get Help
Family Sharing - purchases and payments
Family Sharing - Hide and unhide music, movies, TV shows, audiobooks, and books
Family Sharing – share iCloud Storage
Family Sharing - Share an iCloud storage plan with your family
Family Sharing and location
