Hello,
To lock a device your child is signed in to:
Go to the Family Safety settings for your Microsoft account and make sure you’re signed in as the family organizer.
https://account.microsoft.com/family/home
- Select your 'Child Account' on the Family Safety settings page.
- Under 'Devices', you should see an option: 'Lock devices'.
- Select 'Lock Devices'.
- You can then lock the device to give them time away from their screens. The device will remain locked until the end of the day.