If your child already has their own Google Account, you can add supervision and manage their parental controls with Family Link. To add supervision to your child's existing Google Account, you need to have a Google Account yourself. If you don't have one you can create a Google Account HERE. Your child or their chromebook must be nearby when you’re adding supervision to their account.
Login to the chromebook with the child's Google/Gmail account
On your child's Chromebook, open Settings Settings.
Next to People and then Parental Controls , click Set up.
Follow the steps to setup supervision on the child's account. Your child will need to give their consent during the setup process.