How do you add another device on family sharing for the same kid?

Family Sharing is tied to the Apple ID, not specific devices.

As long as the Apple ID that is part of the Family Group is signed into the devices, they can be used in Family Sharing directly. There's nothing else that needs to be done.

Just make sure both devices are using the child's Apple ID for iTunes & App Stores, and iCloud, and Family sharing should automatically be available.

See here: Sign in with your Apple ID - Apple Support

Hello and welcome to Apple Support Communities, Dukes_Family19.

If I understand your post correctly, your child has one device with their own Apple ID already and you're trying to add a second device for them with the same Apple ID. I’d like to help.

They'll just need to sign in on the new device with the same Apple ID.

Follow the steps in this article:

Help your child set up an iPhone, iPad, or iPod touch

Let us know if that resolves this issue for you. 

Thanks for using the Apple Support Communities. Take care.