I'm an artist IRL, and I have also been looking for good art curriculum. I can do it, but I am not sure how to teach it, especially teaching it to my kids, if that makes any sense. I was impressed with Mark Kistler's Draw Squad. I haven't used this personally, but I ran across it when researching, and saved it in my Amazon Wish list. It's got great reviews, and it looks like it's a well done drawing course for kids...and even adults are using it to improve their drawing skills.
Here's one of the reviews from a teacher:
"As a fourth-grade teacher who was never very good at art, I always found teaching art to be very difficult. Elementary school teachers aren't provided with any real curriculum, and the majority of ideas out there in books and teachers' magazines are simplistic cut-and-paste busy work that don't really teach any art concepts. Then I found this book! This is incredible! My whole art program is set up around this book! It teaches art concretely and step-by-step so that the most art challenged students (including myself) are able to learn from it! The kids love it, and its great for teaching mathematical and geometrical concepts like shapes, angles and terms. I couldn't recommend this book for any teacher (or child, or parent) more! You and your children will draw great pictures, even if you are as bad at drawing as I am!"
Once your kiddos master the basic concepts (shading, contour drawing, composition, etc.) I would move them on to an area of interest or study for drawing inspiration. Most art/craft stores have a drawing section full of 'how to draw' books that walk your child through the steps of drawing horses, dogs, etc. The instructions are sparse in these types of books, so they would greatly benefit from already doing shading and drawing exercises before diving into these.
My husband was taught to draw using the Audubon book of birds. He developed a life long love of birds, and learned a lot while looking at the birds and reading about them. I think we're going to incorporate sketching from the Audubon book when we are doing the Apologia Biology I next year. Adding in an illustration to writing assignments is another fun way of keeping them engaged with drawing.