Well, I am certainly no expert on the subject and I can find little info on what to do with my own artistically gifted child! However, I can say what's worked for us.
My daughter is just a little older than yours as well, she just turned 6 yesterday.
As far as materials go, I try to make sure that anything we have available for her to use is good artists quality. It can be incredibly frustrating to find that the crayons don't colour in smoothy or that the markers bleed. The right kind of paper for the right kind of tool makes a huge difference. I think part of that is mine is also a perfectionist though.
We make sure that materials are always available and organized. So, we have a box for crayons, one for paints and accessories, one for pens, piles of paper...
I don't know if your have pastels or not but they were a revelation to my daughter who loved that the colour was so bold and immediate and that they are a really flexible as they can be blended to shade as well as combined and blended. Regardless of all of that, you may find that yours starts to gravitate to one particular medium. For the last year my daughter has only done ink drawings with no colour.
Beyond the materials, most important for us is making sure that there is always space to create at a whim.
Our favorite art books are The Art Book For Kids and book two
of the same.
They are fabulous book all about art and the artists with text that is child friendly but not at all condescending, they give real insight into how art is made and what's behind it all. I really like that children are expected to give their opinion on the pieces and that no answer is "correct".
These books really helped what my daughter was frustrated at not being able to do specific things perfectly and also when she came to the realization that she was talented (she started comparing her work to other kids who she felt didn't measure up). It helped her realize that there are all kinds of ways of creating art and that there is no right way of doing it.
Aside from that we also really like 'Lives of the Great Artists'
There are suggestions with each artwork on things to do that relate to that particular piece. They are mostly ideas rather than instructions. An example would be; in the section about Turner who painted amazing skies the suggestion is to create a painting of the weather outside.
I'm really not keen on things that give detailed instruction and my daughter seems to respond best to inspiration and ideas rather than specific projects. She also seems to work out emotion through art so we do try and encourage her to draw if something was exciting, sad, unusual etc. through the day.
She has been to art classes but I think the value in that can really depend on the actual class. The ones that she went to were held at the university in the art gallery. So, while they were meant for kids, they were small and offered technique and inspiration rather than instruction. I was really impressed that they used proper art materials as well and the kids did etchings, print making, sculpture, learned about colour (hot-cold), learned about design and spent a lot of time looking at the art in the gallery and learning about what was behind all of that.
We spend a lot of time at galleries and museums and even have gone out of our way driving for almost 2 hours to see a Henry Moore sculpture that sits on a hill in the middle of nowhere. It was a great trip in fact because it was something that you could touch and feel and the scale of it was huge to her. There was also a great story about how Henry Moore was told that he was doing it all wrong as the classics were what was taught at the time.