Hi Elizabeth,

I know it's been a year and a half since you asked this, but I still wanted to answer -- and to learn what you chose to do.  

My advice would have been to tell them up front that you're going to start out really easy, then everyone would be able to move forward as quickly as they wish. 

From simplest to most complicated, I'd recommend Arithmetic, Pre Algebra, Algebra Basics, and Algebra I on this list:

https://www.khanacademy.org/math

(It's changed a bit since you first asked the question; Intro to Algebra is now gone)

I've found that it's better to start easy and quickly move up in difficulty than to start hard, run into a wall, and have to drop back.  If nothing else, the easier early work will get the students used to how Khan Academy works.

If you started with Arithmetic, for example, I'd have your students go straight to the Mission (rather than trying to work through the lessons methodically and start their "Mission Warm-Up". Emphasize that when they're doing this that getting continuous correct answers on the Warm-Up and the Mastery Challenges that follow will allow them to finish much, much faster.  Also, work that they complete on these skills will also count towards later Missions.

I hope it all turned out well for you -- let me know, if you can!