Let's start with the walls. Look at your child care area. Chances are you have lots of primary colors on the wall as well as commercial characters from Disney, Sesame Street, Barney, etc. It seems child-oriented, but is it?
Think of all the marketing targeted to preschoolers through their favorite characters. There are videos, clothing, shoes, dishes, toys, books, etc. The kids are bombarded with this stuff all over. Well, I took a stand and will have much less of that on my walls! I'm working on weeding commercialism out of the toys and books and videos, too. I can't control all of their lives, but I can control what they are exposed to during the time they have here.
So what is on my walls? I'm currently looking for posters of real art. You know, posters of paintings by famous painters. For now I have a flag poster with the Pledge of Allegiance, a few preschool posters (colors, numbers, and sign language), the children's art, and a poster of desert flowers. After taking that other stuff down, it's much warmer and more inviting.
Lighting is another area to work on. Try for as much natural light as possible. Lamps are good, too. Stay away from fluorescent lighting.
Use as much wood and wicker as possible. Try to stay away from bright-colored plastic stuff. I know this sounds contradictory, but these changes can soften up the mood of things. Use wood-toned shelving, tables, chairs, etc. Switch to wicker baskets for storage of art supplies, small toys, etc. Thrift stores and yard sales are great for this.
Bring in fresh flowers as much as possible. Even silk flowers and trees are good. Try to stick with earthy stuff. You will be surprised by the difference and the calming effect it has.
You can take a look at the playroom here.