I used Thinkwell's Prealgebra course with my 9yo son. The lectures are great, but the problem sets are terrible. A well constructed problem set takes a student from what he knows through what he doesn't know but is learning by doing the problem set in a fairly seamless manner (Jacobs is the master of this, and Singapore is also very good). The Thinkwell problem sets are not varied enough nor do they develop a concept in a systematic way. Not recommended.