1. Közösség
2. Content Requests and Feedback
3. AP courses

I second this request

1. Közösség
2. Content Requests and Feedback
3. Why are there no psychology/psychology related courses/classes?

There are. The MCAT course section has a ton that helped me ace my sociology and psychology courses in college. YouTube has self-made Khan Academy playlists for psychology and sociology. 

1. Közösség
2. Content Requests and Feedback
3. Grammar Grade Levels?

Khan Academy Kids is a great start and then the grammar course here is very strong once they're ready. I would say a third grader can do it, but it may help to have adult support to make it fun.