You do not have to complete every part of every assignment. There is no possible way to do it all. The only thing you need to do is meet the objectives for the lesson. Once you meet the objective (this may be done by prior knowledge), take the short assessment at the end, mark it complete and move on. The objectives need to be met, and students need exposure to many subjects. That way, they can learn what they are passionate about and what they are not interested in (great for choosing a career later) and there will be no gaps in learning when they are in the rigors of high school, college or grad school.
Children’s minds are like sponges, they are created to learn. Expose them to rich materials and lots of topics, subjects and information. K12 does an excellent job of this. It stretches us as moms and parents and has our students learning a wealth of information about topics we would possibly never introduce to them.