One of the best ways to get parents to understand the development of young children is through talking with them about it and then providing resources for them to read so that they can learn more.  If you are a preschool teacher, consider offering an after-school presentation on this topic for parents of the kids in your class.  Let them ask questions and give them specific examples of what they can expect during these preschool years.  In the link I've attached below, Education.com offers a whole section of their website devoted exclusively to 3 to 5 year olds. You might find that there are some articles that describe exactly what you're wanting to help parents understand.  You can either tell them about this resource or print out some of your favorite articles and hand them out.  

Best of luck!

Polly Dunn, Ph.D.
www.ChildPsychMom.com
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