It seems to me that also if you succeed in making child-only spaces, those spaces become a magnet for adult abusers. They become an all the more desirable prize for them. Whereas spaces like this - hacker news, that is - don't need any age verification because although it's a safe bet some users are underage here too, the abusers would have to search a long time for them and the seemingly most common manipulation techniques (like pretending to be a child yourself) probably wouldn't work.