My child phone has been stolen so I’m trying to delete it of my family sharing how do I do that

See If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support. In general you want to block the phone, but not remove it from your or your child's Apple account. That stops the thieves from resetting the phone and selling it on. You would leave your child's Apple ID active in Family Sharing, but perhaps change their password as a precautionary measure.

tt2

Just to clarify Family Sharing, does not apply to specific devices. It applies to Apple IDs. Removing the child from Family sharing as suggested above will remove all of the child’s devices from Family Sharing which may not be the desired outcome.

I would as suggested by Turingtest, set the device to lost mode, and keep the child’s Apple ID in Family Sharing, so other devices they may have and any replacement iPhone can continue to access the Family Sharing purchases and content.

Go to settings>click your name>family sharing

click the name of your child and then remove from family sharing