Your channels are connected to your account, not your individual devices, so all devices on the same account see the same list of channels on the home page channel grid. To have a different list on the kids' set, it would have to be attached to a different account.
If you log into your account on my.roku.com then go to
https://my.roku.com/account/pin, you can choose a PIN number and set where it must be used, including parental permissions.
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