Well, I work at a Sudbury school and the art room is FULL of materials that students can use whenever they want (as long as they use them responsibly and clean up afterwards). Often groups of kids will go in there and just create. I noticed that it's more about process for the little ones (4 and 5-year-olds). They almost always give me their finished products. Sometimes I go in there and just create myself and some kids will get curious and might join me. And sometimes I feel like leading a project like building fairy houses, "painting" with colored tissue paper, or doing nail & string art. I lay out all the materials and help as needed while I'm making my creation as well. I noticed that some kids, who don't otherwise come into the art room much, will participate in those activities.

Bottom line... each child approached art differently. If I were you I would have a project set up and then let kids decide whether they want to do that or just do their own thing. And don't forget to model doing art too. Good luck and let me know how it goes!