Art lessons, lots of them. I was fortunate to have a high school with an excellent art dept., so I was exposed to a lot of different things. If your district is not as well equipped, I would seek out qualified art teachers for extracurricular lessons.
The emphasis for her now should not be styles of artwork, but mastering basic techniques and learning to work in different media. Hopefully, you can find art teachers who already know this stuff and are able to expose her to a lot of different methods of creating art.
Some things:
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live figure drawing; probably the single most important thing any artist can do, in my opinion. The format of these sessions provides a tremendous opportunity to learn about so many things: proportion, space, shape, line, light; plus, you get in the habit of drawing A LOT. That was one thing that caught my attention in your OP: “The drawing she produced in her first lesson was pretty amazing.” In my opinion, a learning artist would be better served by making dozens of extremely quick drawings in a session, so she can (a) learn to see and work on all parts of a drawing at once–rather than getting tunnel vision on any small portion of it–and (b) learn not to treat any single drawing as precious. If she’s going to be an artist, she’s going to make thousands of drawings, most of them awful, over the years. You have to throw out a lot of drawings to find the ones that are good.
the figure drawing sessions are also a great occasion to explore different media: pencil, marker, charcoal, chalk, pastel, crayon, paint
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sculpture - especially if she’s interested in computer animation (which nearly always means 3D animation), she would be well served by learning a good deal about sculpture. It’s a huge field, and beyond my specific experience, but just getting started and getting familiar with crafting three-dimensional representations of things, etc., would seem essential
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painting - the specifics of mastering painting techniques – how to wield brushes, how to prepare a surface, how to mix colors, etc. – is important for a young artist to learn, but more important is probably the color theory/knowledge/aesthetic sense she’ll pick up in the process. Also, painting allows you to approach art differently than drawing: you can render entire shapes very quickly, instead of just the lines that define those shapes
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photography - photography is a great tool for learning more about light and more about composition. The best thing about it is that (esp. with digital cameras) it’s fast, and easy to capture lots and lots of images. So, whereas composing a drawing might take a while, you can test out several dozen different compositions in minutes with a camera. And, again, you approach it with the idea that you’re going to throw out 90% of your photos, so no single act of creating art becomes too precious.
There’s a lot to learn, even just to get broad exposure, but the basics, in my opinion, would be live figure drawing, painting (esp. to learn how to deal with color), some introduction to theories about perspective (hopefully covered along the way), and, since she’s interested in computer animation, sculpture.
But above all: QUANTITY. The only way to improve is simply an insane amount of practice.
