I just found out about The Artist's Toolkit. It's an easy to navigate website that teaches art elements and principles like line, symmetry, tint and shade, etc. Each lesson has three sections:
Watch an animated demonstration
Find examples of the concept in works of art from museums
Create your own composition
There are also a few videos of artists in action and an encyclopedia which illustrates the explanations using paintings. I really like how they incorporate actual art into the lessons. For instance, we did the lesson about cool and warm. One of the paintings they used to teach about the concept was:
Van Gogh--Olive Tree
I would recommend for younger children, you watch with them. Mija really enjoyed it, but it helped that I was there to bring to mind the principle we were learning. That's all I did; the website did the rest!