How do I add my kids new phone on family sharing

Start here:

If they are under 13 => Create an Apple ID for your child - Apple Support

And here => Set up Family Sharing - Apple Support

Devices are not themselves added to Family Sharing. Apple IDs are added and any devices logged into said Apple IDs will be able to access the Family Sharing purchases and items,.

If they already had a device logged into their Apple ID and their Apple ID was already part of Family Sharing, then just logging into the Apple ID on the new iPhone would be enough to let the device access Family Sharing.