The PIN is tied to the Roku account the device is linked to, so the only way to have devices with different PIN settings is to link them to different Roku accounts. Note that subscriptions through Roku are also tied to the Roku account, so if you subscribe to anything through Roku you wouldn't have access to the subscription on a device linked to a different account. Subscriptions taken out directly with the provider (e.g., Netflix, Paramount, etc.) don't have this limitation which is just one reason why it's generally better to subscribe to something directly with the provider.
You might also want to look into Guest Mode. I don't know whether or not it would work for you in your situation.
How to turn Guest Mode on and off on your Roku® streaming device | Roku
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