Hi hueyyueh1, the World history material is intended for High school and to some extent, Advance placement classes in High school. Some middle school students may benefit from it, but I believe most Middle school students would find it rather advanced had have too little basic background knowledge to appreciate the content. 

There might be videos that can support a Middle school curriculum and that you find suitable for your child, but as you know your child best it is always recommended that you watch the videos yourself before recommending them. 

Currently you cannot user Khan Academy tools to make recommendations for videos. This is something that Khan Academy is looking into creating as the feature has been requested by many teachers, so perhaps you will see it later. For now, creating a play list with cut-and-paste links in another document or email is the easiest way of making video recommendations.

I hope this helps.