Be selective in which iCloud services it uses. Also set up ScreenTime so your settings can't be changed. Use Screen Time on your iPhone or iPad - Apple Support
Read this support article about how to change your iCloud feature settings [iOS and Mac] - Change which apps sync and store data with iCloud - Apple Support - "'When you turn off an iCloud feature on a device, information stored only in iCloud for that feature is no longer available on the device. If you turn off Mail, Contacts, Calendars, Notes, Reminders, or iCloud Photos on just one device, you can still view and change your information on iCloud.com and on another device you set up for that feature."
Note that in general Apple's i-devices are designed to be used by one person at a time so you're kind of working against how they are designed to be used.