I don't think this is as complicated as you're making out.
The diagram is pretty simple
View attachment 320933
Whilst we all answer yes for stage one, a lot of forums can easily answer no to both questions in stage two and thus not need to do a child assessment.
It even gives a load of forums as examples at the end of the document.
If your forum is unlikely to be of interest to children, all you need to do is log the date, your reasoning of how you reached that conclusion.
I'd say automotive forums are pretty safe there, but I can see why a hamster forum might fall foul.
Then we just keep it filed? Do we have to actually submit anything anywhere?