One of my favorite age groups to work with is preschoolers. They are so curious and enthusiastic about new experiences; I love watching their brains work and evolve. I find it is imperative to encourage open ended projects, with a focus on process rather than end product.
Kids are hard at work every day building their skills, and art can play a huge part in that process. Art allows for exploration, motor skill growth, problem solving, and working creatively. They are exposed to new materials, which stimulates the senses, and enhances their view of the world in different ways. Working with clay strengthens hand muscles, holding a paint brush increases fine motor control, examining textures on materials enhances observation skills, and assembling a collage provides a puzzle-like challenge.
Directing young children to produce a specific piece of art can be counterproductive to learning, or even enjoying art as a whole. Encouraging exploration and creativity during the art process enhances their experience. This will likely not result in a perfect recreation of Van Gogh's Starry Night, but is that really what a small child needs in order to be an artist? Being an artist is much more about finding your own place and exploring your medium. The experimental process of art is scientific as well as artistic, which allows for more effective skill development. Observation of subjects becomes questioning, brainstorming ideas becomes testing those ideas, and finding the perfect materials becomes a scientific process in its own right. What will work best to show others my ideas? How can I effectively reproduce the images in my head?
Art is always evolving, and each artist has their own way of portraying the world around them. There are no wrong answers in art. Allowing for process to take over is important to let children know that they can make art however they would like. Young children especially can use the freedom to explore how they wish to test their boundaries and take risks that may not be available to them in other parts of their daily life.
Of course this isn't to say a guiding project is off limits. These can be quite helpful in focusing your child, or limiting the purchase of materials. But even in this endeavor, staying open ended and process-driven is possible. Perhaps the idea of presenting a project or famous artwork as inspiration, rather than as a right or wrong "answer" is the way to go.