The advice I’ve received from artists is to hold off on teaching technique as long as you can. The best way to learn to draw is to draw. Keep encouraging them to work on their art projects (and you jump in when you can…it can be motiviating to see mom struggle as a beginner). Draw, draw, draw, and also look at other artwork through the eyes of an artist – get inspiration and instruction from what other artists are doing.
If they themselves are specifically asking for instruction, I recommend the book “Drawing from the Right Side of your Brain.”
If they aren’t asking for instruction, I would simply supply them with a set of pencils and a stack of paper and give them time to work and encouragement to create.
One of the things I absolutely love about homeschooling is that the kids have the time and opportunity to work on their art. Go for it, in whatever way works for you and the boys!