How do I transfer a child and join a new family for family sharing?

If you have macOS Catalina

1. Choose Apple menu  > System Preferences, then click Family Sharing.
2. Click the Add button.
3. Ask the child to enter their Apple ID and password. ... 
4. Follow the onscreen steps.
5. After the child's current family organizer approves the request, provide parental consent and a valid payment method.*

Move a child to another group using Family Sharing - Apple ...

If the child is already in your family

You can ask an organizer from another family group to invite the child to their family. After they invite the child, you get a notification. You can approve from the notification or use these steps:

• On your iPhone, iPad, or iPod touch, tap Settings. Below your name, find a notification that says "Family Transfer Request." Tap the notification and follow the onscreen instructions.