If you set up screen time but it isn't working as expected, here are a few things you can try:
Set up one schedule: With the latest update of Windows, you're able to set up one schedule that'll apply to all of your child's devices. This means if you give them five hours in a day, they'll have five hours between their Xbox One and Windows 10 devices. Otherwise, time gets tracked separately, so one hour of screen time means one hour per device.
Restart your device: Select Start > Power > Restart.
Sign in: Make sure your child is signed in with their Microsoft account on their device. If they are, you can check to make sure their account is syncing up right. Select Start > Settings > Accounts and follow the prompts to verify their account.
Keep it in the family group: Accounts outside of your Microsoft family aren’t subject to screen time limits that you set. To prevent kids from creating new accounts, on your Xbox press the Xbox button, then select System > Settings > System > Signed-out content restrictions. Create a guest key and turn off Let people download & make new accounts. From then on, you'll need a guest key to add new accounts.