@mora_sd-
Roku parental controls, such as they are, are attached to the Roku account, not to individual Roku units.
To have separate parental controls for you son's Roku you would need to factory reset it and set it up with its own separate Roku account, using a different email address. Keep the password for this account secret if you don't want him to be able to change the settings.
Any paid subscriptions you have through Roku are connected to your current account and will not travel to this new account.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
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