Apple ID for child

Here's a guide as to how to create a child's ID --> Create an Apple ID for your child - Apple Support

The child will need a different email address from the one used for the other account.

Note that it is not possible to merge content from 2 IDs later. The child will have to retain access to both accounts and deal with the complexities of this.

As long as there are separate devices this is fairly simple.

Setup yourself as a family organizer.

Create a new AppleID for your child using a slightly different name (e.g. Jim vice James; Kate vice Kathryn)

Do NOT use a fictitious birthdate; it’s difficult to change later.

Then, setup the child’s device yourself. Pay VERY CAREFUL attention to the questions being asked.

Set up Family Sharing - Apple Support

Create an Apple ID for your child - Apple Support

Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

As a minimum, would suggest using Parental Controls (and Screen time) to prevent your child from changing his/her AppleID account settings or device passcode.)