Not my post on Apple Support, but essentially the same issue (quoted for ease of reference):
I have got the Apple TV app running on an Amazon Fire TV. I have attempted to set up restrictions so that my youngest child could not access age-inappropriate content (both TV Shows and Movies).
The restrictions for Movies works as expected – when you try and open a Movie with a rating above the restricted level, you are prompted for the passcode.
For TV Shows when you try and set restrictions for the UK, you are only presented with ‘No TV Shows’, ‘Caution’ or ‘Allow all TV Shows’ as the level of restriction. It seems that the only rating of TV show for UK audiences is ‘Caution’ so in practice, this means that either all TV Shows are blocked and do not appear in the app or all TV shows are visible without requiring the passcode.
In my case I am in the UK and have just installed the AppleTV+ App on a Roku. My Roku is intended for use entirely by small children. I don’t want adult-rated content to even appear in the catalogue that they may browse. This misfeature is an absolute show-stopper for me. If I am unable to set parental controls for all content then I shall simply be cancelling. There doesn’t seem to be an option to contact Apple for support, can anyone here advise?