How can I shut my child's xbox off from my account

I don't think you necessarily can without being logged in already on the console. If you have your Xbox paired with the Xbox App or Xbox SmartGlass (if that's even relevant anymore...) you may be able to do a remote turn off there.

Child accounts or accounts added under a family on the console can have play timers set up on them in the settings menu if you're on the host/parent account for that family/child. Once that timer goes off it should immediately sign the account it was put
on out, or something similar to that. If you look it up there should be a guide on Xbox.com

You can manage how long your kids spend playing games and watching shows by telling Xbox how to manage their screen time.

Set screen time limits online

1. Sign in to your Microsoft account.
2. On your family page, select Screen time under your child's name.
3. Here you have two options:
   • To use the same schedule for all devices, switch Use one screen time schedule to
     On.
   • To set up separate schedules, switch screen time for PC and Xbox One individually to
     On.
4. You’ll see each day of the week listed. You can either:
   • Customize how much time your child can spend on their devices each day, and when they can use them, or
   • You can give them the full amount of time you’ve scheduled by selecting
     Max scheduled. (For example, if you’ve allowed screen time from 8 am to 8 pm, they could use their devices that entire time.)

Set a screen-time notification on your Xbox One console

To give your child a heads-up on the console when screen time is about to run out:

1. Press the Xbox button
   to open the guide.
2. Select System > Settings > Preferences > Notifications.
3. Select Xbox notifications > System, and make sure System notifications is set to
   On.

If your child runs out of play time, you can add more by signing in to your account on the console. When you’re prompted to extend your child’s time, choose how much extra time you want to add.

i think you should find this link very handy, to help you look into and set up family settings. https://support.xbox.com/en-GB/browse/xbox-one/security/Family settings

Are you looking to just modify permissions or you actually want to terminate their account?

The family security is a good place to start, setting screen time etc. You can also do things like, take his controller away or the power lead for the console. Another option, that I did, was to set up times within the router so as to shut the internet
off to that Xbox.