See my reply above. Content designed and marketed to children nowhere else has 18+ advertisements. Same standard should (and has to imo) be applied to "new" media.
That's because nobody else (in the streaming video space) is trying to serve ads based on who's watching the content. YouTube is in a different advertising market that nobody else has yet attempted to enter.
When it comes to children's programming...it shouldn't be based on who's watching, it should be based on if the content itself is designed for children or not and serve ads accordingly.