How to stop my child from creating new accounts on Xbox One?

So I finally found this. If you go into settings, you can select "system" settings. In there you can select "control offline content". In that setting you can uncheck "allow new profiles" and also set the age level for access when no one is signed in and create PassKey to prevent changing those settings. That will control what can be done when no one is signed in and also prevent creating a new profile without the passkey.

All the talking in the world doesn't help some kids.

What you have to do is set up a guest profile with pass key. Then uncheck the box allowing creating new accounts. This will prevent new account creation when not signed in.

Next, make sure your kids' profiles are under your family and then lock down their account how you choose.

This worked on my system.

Something you can do, is stop sharing gold on the account. Xbox One allows gold sharing, which means only one account on the box needs gold for all accounts to benefit. You can find the option in account settings somewhere(sorry, I don't know the exact location... I've never had to turn it off).

I don't know how well this will work, and the kid will be able to just re-enable sharing once he figured out what you did, but it may stall them for a little bit at least.

Overall, the comments are 100% correct. Talk to him about why you are blocking the content in the first place, and be sure yourself when he states his case about why he wants to access it, that's just as important that you listen, and try and work out a compromise.

I am in the same position you are. The only guarantee I found that works to keep this from happening is I took the controller away at night.

As far as the talking to your child thing. It sounds like a great idea. Unless you actually have children. Then you would know that this almost never works. As soon as your back is turned they go right back to doing it again. If you have a child that is responsible enough to trust to listen to you, they probably wouldn't be doing this kind of stuff in the first place.