New Game Rating: Kid-Friendly
In fairness, there is a difference between "suitable for all audiences" and actually "kid friendly". It's actually a pretty big difference. To me, that's less a matter of rating, and more, is the GM interested in a younger player group? Some people don't like playing with kids, while some games are actually aimed for it.
If there was enough use for it, I think it could be a useful thing, but I would suggest it as a Category, not a Game Rating. I know kid - everyone - adult is a nice spectrum, but as BBR notes, a KF Rating wouldn't actually impose more limits than are already there, and people have wildly differing ideas of what is appropriate A) for kids, and B) for their kids in particular. Also, C) who counts as a `kid', for that matter.
As a Category, it would be an easily searchable way for a GM just to indicate that this game is looking for younger players. And since you can have three Categories, there's still plenty of room to describe your game otherwise.
What I don't know is if there are enough kids on RPoL to warrant it.
I helped my daughter set up an RPoL account when she was nine, which I think would count as a "kid" for about anyone. I remember particularly because we needed some help from Shannara, who, I have to say, made sure I understood the policies, but never said anything, you know, judgy or discouraging, like, "Are you sure a nine-year-old is ready for this?" So thank you for that. grin Her interest faded after the game she joined for died, but it was a really good experience for her, helped her reading and writing quite a bit*, and she has the account if she wants it. I certainly never regretted it. But, even though she could be, she's not actively on RPoL anymore.
For the most part, and noting that an old rPoll puts our `arguably-a-kid' demographic at only 6.9%, I think you can usually tell pretty easily which games are kid friendly and which ones aren't. Or, more to the point, given the rules, kids can tell which games look interesting to them... just like everybody else. And nobody actually has to know they're a kid if they don't want them to. At least until they start talking about their hot new fidget spinner, of course.
*- Which were already good, mind you. I wouldn't have dropped her on some poor un-consenting GM if I hadn't thought she could keep up.