how do i set up my old iphone for a kid

See this Apple support article for resetting the phone—>What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support then create a new Apple ID using the phone for your child. You can create a Family Sharing group if you want to manage your child’s phone—>Family Sharing - Apple Support as well as share content (selectively) among family members.

Here’s information on parental control options—>Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

Well, for starters, I would not recommend using a iPhone for a kid. First of all, it has cellular data meaning it can call people using phone numbers, although it depends on the age of the kid.

You might want to settle on a iPod Touch.

iPod touch - Apple

If you still want to settle on a iPhone, use parental controls.

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

REMEMBER, erase the phone first.

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

In addition got what Lawrence Finch said, see this article (if your kid is under 13):

Family Sharing and Apple ID for your child - Apple Support

And to add even more, don’t lie about a child’s age. It can’t be changed, and will come back to haunt them in the future.

Yep. Big consequences. It can lead to apps rated older than the ACTUAL kids age, content that’s not appropriate for their age such as websites, apps, music etc.