How do I give my old phone to my kid?

See what to do before you sell or give away your iPhone --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Once you've done what is recommended, you can set up the iPhone for your kid, with their own Apple ID, etc. If they are very young and you want to use parental controls to have more control over what things they can or can't do, this link will explain how to do that --> Use parental controls on your child's iPhone, iPad and iPod touch – Apple Support (UK)

And finally, you can use Family Sharing to share apps with your kid, so they won't have to buy things you already own and this link explains this in more detail --> Family Sharing - Apple

You're very welcome.