Much of the content in the Arts and Humanities section on Khan Academy is also included in Partner content. Some examples of partner content here include the music section and content sponsored by various art museums. The history and Art content contains quizzes where speed badges are available. The music content contains a lot of long videos. Most of the art content is probably not appropriate for children, though it is accessible for the child accounts. The Art History Basics section should be ok for kids, though younger kids may find it boring. The Getty Museum is also a good area for kids to learn about art.
In January of 2016, Khan Academy began to roll-out their frequently requested and much anticipated grammar course.
David Rheinstrom teaches users The Parts of Speech beginning with Nouns: people, places, things, and ideas. As he mentions, ideas are often lost on others teaching about nouns.
Learn more about this here:
https://www.khanacademy.org/humanities