Removing Google Family Link
Google's Family Link ties itself to your child's age. As soon as they turn 13, they automatically get control over their Google Account which results in disabling all controls imposed by Family Link. In fact it gives the option to your child to continue being monitored or not - they have to give consent to their parents. Google will email both the parent and child about this change prior to the child turning 13.
When you delete your child’s Google Account while they are still under 13 years of age, their information associated with their account will also be deleted. This includes emails, documents, and anything else associated with it.
1. From your child's device
Open Family Link, tap the menu icon (top left), tap Remove Account and confirm. You will be required to choose your parent Google Account and enter your password to confirm this step. If you child's device doesn't have an active internet connection at the time, you can use a code generated by from the Family Link app on your parent device.
2. From your parent device
After completing the child device steps above, open Family Link on your parent device.
Tap Manage settingsAccount infoDelete account and follow the on-screen prompts.
Undoing the removal of Family Link
Google seems to allow a way to undo the deletion of your child's account. You need to visit their Google account via this link: https://families.google.com/.
Then select the Menu Recover kid's deleted account Recover.