Great question! Personally, I had to make sure my sibling was practicing exercises on Khan Academy, and had many of the same questions as you. 

In terms of the points workaround, and making sure a child is not doing exercises just for the points, is to monitor how many points your child earns, and in what time frame. After my sibling was done practicing, I would go to their Activity section to view this. You can do this by going to their Dashboard, and clicking Progress, as highlighted below:

There, you can view all the progress your child has been making during a specific period of time. In the progress section, click Activity or Focus for a detailed breakdown of what your child has been practicing, for how long, which videos and activities they've completed, and how many energy points they've earned.

So, if you want your child in Grade 4 to do 30 minutes of practice a day, you can make sure they are completing that, and you can also see if they might be deliberately extending the time for exercises. Usually, exercises only take 2-4 minutes at most, but if you see that they are taking longer than that, you could point that out to your child and ask them about it. 

Those are really only the strategies that I used to monitor my siblings progress, and that will likely work for your children. If you have any other questions, please ask and I can clarify anything.