The TV app walks you through the three permissions it needs to enforce anything. Each opens an Android settings screen — say yes, then use Back to return to the checklist. Then set your parent PIN.
Android sometimes buries these under Settings → System → Accessibility or Apps → Special app access. The checklist links straight there, so follow it rather than hunting through settings yourself.
Pick a PIN your kids won't guess — not a birthday, and not the same code as the TV's own lock. It's stored on the TV only as a salted hash, so keep a copy somewhere safe for yourself.