Sadly, your daughter's preschool is doing what many others do today. They've co-opted art portfolios from elementary schools who've co-opted them from high schools who've co-opted them from art schools. They did this because parents get easily impressed by portfolios, thinking they sound “grown up” and “sophisticated” and will give their child an early edge. Art portfolios, though, aren't developmentally appropriate and have no place during this time when children should be doing open-ended art at preschool.
Young children, exploring materials and techniques for the first time, need teachers who recognize that process is far more important than the product. Open-ended art at preschool—drawing, coloring, painting, printmaking, sculpting, and collage making—need to rule supreme while teacher directed craft projects should be minimized or even eliminated. Having high-quality art projects to place in portfolios so parents can ooh and aah at them at the end of the year destroys children's drive to express themselves through art.
When considering early education opportunities, parents should look for those that celebrate open-ended art at preschool. There should be easels (inside and outside), crayons, colored pencils, markers, and collage materials that are readily accessible, time throughout the day to create, and play-dough for children to pound, roll, and mold. When I visit preschools for my job, I love to see “creation stations.” These are a corner of the classroom set aside for kids to make sculptures from recycled materials: cardboard boxes, packing peanuts, straws, bottle caps, plastic utensils, etc. This is preschool art at its best as it promotes initiative and imagination.
If your daughter's preschool does not emphasize open-ended art, you have a tough battle in front of you. Unfortunately, many preschools today are uptight places that see their primary function as preparing kids for kindergarten. With that narrow mindset, creative expression gets put on the back-burner.
I'd suggest you talk to your daughter's teacher and let her know how much you value open-ended art at preschool (this is probably something she rarely hears). Tell her why you see it as important. Ask her how more can be incorporated in the school day. Most preschool teachers have no background in arts education and just pick it up as they go. Your questions and concerns will get her thinking about her philosophy on art education and, hopefully, get her to see the immense value of open-ended art at preschool.
Good luck. I admire you for recognizing how important art is for young children. As we see more mental health issues appear in children and teens (depression, anxiety, suicide, narcissism, a lack of empathy), it's important we realize that this increase corresponds to the decline of play and creativity at preschool, kindergarten, and at home. Too many parents see art as frivolous—something that will never lead to a job—but they're way off base!