want to set up my old iPhone 6 for grandchild

The first thing you need to do is the steps in the article below to make the phone 'their' phone.

Selling/Giving Away Device.      

You need to create an Apple ID for them.         

If under 13 years old, you will need to set up Family Sharing to create an ID.

Family Sharing and Apple ID for your child.      

Family Sharing.         

Family Sharing - Set up.        

See the Apple Support article(s) below.

Set up - your child’s iPhone, iPad, or iPod touch