Old iPhone for games only
The officially supported way is to use Family Sharing. You create a child Apple ID using your account, and you will maintain parental control over the child account, including App Store, iTunes and Apple Books purchases. It will have access to all your apps and music, and you will have the opportunity to approve any purchases your grandchild makes.

Create an Apple ID for your child


  Use the steps below to create an Apple ID for your child, then add
  them to your family group. After you add your child to your family
  group, they will have their own Apple ID that they can use on any
  Apple device. Your email address is the rescue email address for your
  child’s account, and you can use it with the security questions you
  provided to reset a forgotten password.
  
  If the child already has a Game Center account, but not an Apple ID,
  you don't need to create a new account for them. Just search for their
  nickname instead.



Go to Settings > [your name] > Family Sharing > Add Family Member >
Create a Child Account > Next. If you're using iOS 10.2 or earlier,
go to Settings > iCloud > Family.
Enter your child's birthday and tap Next. Be sure to enter the
correct date.
Review the Parent Privacy Disclosure and tap Agree.
Enter the requested information for your payment method and tap
Next. If you don't have a payment method on file, you need to add
one.
Enter your child's name, tap Next, then create their Apple ID
(username@icloud.com) and tap Next. Tap Create.
Follow the onscreen instructions to set a password, choose security
questions, and set up your child's account. Choose passwords and
security questions that you can both remember.
Turn on Ask to Buy to approve all iTunes, Apple Books, and App Store
purchases initiated by your child. You will be responsible for all
charges to your account. Tap Next.
Review the Terms and Conditions. Tap Agree.


Source: Family Sharing and Apple ID for your child

Now you should be free to set up the iPhone 6 with the new child Apple ID. Either:


reset the device Settings > General >  Reset > Erase All Content and Settings then sign back in with the new ID; or
sign out of iCloud, iTunes and App Store, FaceTime, iMessage etc. then sign back in with the new ID.