contact import/export
Family Sharing, as suggested by @Tetsujin, is a perfect option for a child device.
It allows you to:


Share purchases
Set limits to purchases made by the child, or let they ask for purchases and approve remotely
Share Apple Music or music purchased from Apple
Share and see other family member's location on Messages or Find My Friends
Use Find My iPhone for all devices
Activate Restrictions on iOS and Parental control on macOS
Share calendars and Photos Albums 


And it lets you fully sync your own data between your devices, because the accounts are independent.