They must accept this before the family settings will take effect, so make sure they do so. Your child will receive an email with an invitation to join your family group.
You can create a new Microsoft account using their existing email address, or make them a new account. If your child doesn't have a Microsoft account, click the Create one for them link at the bottom. Complete the CAPTCHA, then click Send invite. Select Member and enter the email address or mobile number tied to your child's Microsoft account. You'll then see a prompt to add a user to your family. If this is your first time setting this up, click the Create a family group button.
Sign in with your Microsoft account to continue. You can set up a new child account on your PC, but it's a bit easier if you click Manage family settings online to open the Microsoft Family Safety page. To start, head back to Settings > Accounts > Family & other users. For that, you should try Microsoft's 'family group' feature in Windows 10. But they don't account for the unique challenge of keeping kids safe when using a computer. Standard accounts are great when you don't want other people messing around with your PC's settings. Take Advantage of Microsoft's Family Group