So you've started using Khan Academy with your students and find your students are not doing what you expected. You want to restrict their work to only what you are doing in class.
Khan Academy is about engaging students at their individual levels. It's about focusing on meaningful content. It's about teaching students to become independent learners. It's about exploring a world of subjects beyond math. As teachers we know that we can't teach in a lockstep beat that only meets the needs of the students in the middle, leaving out the students who struggle and the students who grow bored because they get it quickly. We also know that different students have different interests and by allowing students to explore their interests and finding ways to teach to their interests we have a better chance of reaching them.
Instead of asking to restrict what they access, consider the tools you have available. As teachers, and coaches and parents, there are powerful tools that allow you to recommend skills and monitor work. You can set due dates, but then let the students explore when they've done what you've recommended. You can use the tools to allow you to find gaps in learning and use teaching strategies to reach the slower students. You can recommend challenging work for those students that go quickly through your recommendations. However all of Khan Academy's content is freely available for any learner to access if they have the drive to do so.
And of course, there is the powerful tool that is outside of Khan Academy: you! Through having a personal conversation with students you can help them understand what they need to learn in order to achieve their goals. You can instill in them the confidence to keep trying. Another part of Khan Academy is the belief that if you take the time and effort and work one step at a time You Can Learn Anything. Give your students the tools they need to learn anything.