I have done all of these steps and my son can still access YouTube by using the voice search. It automatically opens YouTube even though the app has been removed.
Hi, @Chucking
Extending a hearty welcome here in the Roku Community.
We appreciate you for sharing this information with us, and we'd be more than glad to assist. We understand that this is an unusual occurrence with the Roku device, and we'll provide a troubleshooting step to resolve this issue. Kindly perform a system restart by navigating to Settings > System > Power > System Restart (If you do not see a Power submenu, skip to the next step).
Once you've performed this, kindly check if you're able to see the same issue occur.
Let us know.
Best wishes,
Kash
Chucking-
When you tell it to load a channel app that is not installed, it pops up a screen asking if you want to install it, which of course your son then does.
If you log into your account at https://my.roku.com you can create or reset a PIN value. You can then choose when the PIN is to be required, such as before it will allow any subscription or purchase to be made to your account, or before it will install a channel.