Khan Academy worked well for our kid, who started at that age. She worked through 1-2 grades per year and is now in their Algebra series. The main complaint I have is that there is no concept of 'practice' or 'drilling'. You just learn something and go straight to the quiz.
There are two problems with this. The first is that it doesn't get kids used to doing practice, which is critical for learning more advanced skills.
The second is that the quizzes use a lot of multiple choice (particularly in the younger grades, where kids are not assumed to be literate). Getting a handful of multiple choice questions correct is not a good indicator of having mastered a topic. Once I realized this, I told my kid she had to get all of the questions correct on the quiz 3 times in a row in order to move on.
There are two problems with this. The first is that it doesn't get kids used to doing practice, which is critical for learning more advanced skills.
The second is that the quizzes use a lot of multiple choice (particularly in the younger grades, where kids are not assumed to be literate). Getting a handful of multiple choice questions correct is not a good indicator of having mastered a topic. Once I realized this, I told my kid she had to get all of the questions correct on the quiz 3 times in a row in order to move on.