I'm surprised no one has mentioned Watership Down. Few books have ever absorbed me more than that one, and my first read of it was in sixth grade. It's long, but should be accessible for a 6th grader, especially if she is a little bit advanced. Perfect for animal lovers, perfect for adventure.

Drawing on the Right Side of the Brain is a classic text for adults and has exercises that really help the artist break through the perceptual "wall", but the writing isn't so great (IMO). You would need to explain everything to your children, and in order to do so well, you'd need to do the exercises yourself. Also, the author suggests not starting the book with children until they show signs of readiness, namely, attempting to show depth in their drawings. I like coming up with my own art curriculum by piecing things together, but I have enjoyed using the book Art Lab for Little Kids (not so great for drawing, but good ideas when you want to practice your color theory!). And of course, Google is always your friend. I've gotten loads of ideas simply by googling "elementary art projects" (You've got to weed out all the crafts) and then finding projects appropriate to whatever I want to cover. As for how often, I think that depends on your children's interest and your own interest. If you love it and they love it and you have enough time, then maybe two or three times a week (that's what we do).