As we've seen with Stack Exchange, OpenID is confusing for most users, but good news: you don't need a Google or Facebook account anymore! You can sign up for a Khan Academy account with any email address here:
Off topic, but can you add the ability to check off completed videos instead of having to watch them within the website? I usually play KA videos at 1.5x or 2.0x on youtube, but then I don't receive any points :(
I don't remember exactly, but it was a few months ago. About a month ago we also launched the ability to create "child" accounts for under-13 users who aren't old enough for Google or Facebook accounts.
Sorry, it's not possible, but the main reason to create a child account is to get around the Google/Facebook requirement -- since you've already surmounted that hurdle, a child account probably wouldn't be of any more use to you.
http://www.khanacademy.org/signup