Generally when I talk with parents about learning through play, I do it at the start of their child's enrollment in my infant or toddler room. Since they get the Ages and Stages Questionnaire at their home visit, I get a chance to go over the domains of development and answer questions at that time.
If they ask me about precursor literacy at the home visit, I generally share that children learn these concepts as we read to them, engage them in songs and stories and puppet play, and through the give and take of conversation, as well as by observing environmental print and learning to scribble with writing tools. I share that play-dough play strengthens the muscles that will later be used for writing. I have a piece up in NAEYC's "For Families" website on the topic that I also can share.
Then, in our first documentation piece (usually in week two), I talk about the contents of every area of the classroom and give examples of the kinds of tools children develop through each of the activities in relation to the domains of development (including language-literacy -- I generally begin my focus on that topic in the Quiet Area). My goal is to help families know that children need to develop skills across the domains, and to create home to school links about the importance of learning in all areas, from learning the precursor concepts of math in the block and cognitive areas, to developing social and emotional skills in dramatic play, to learning to use tools such as scissors while developing hand-eye coordination in art-sensory.
I find that documentation is hugely valuable -- pairing an image of the children at focused work with a description of what they are learning in each domain, why it is important, and connections to a place for further learning such as NAEYC for families, or Zero to Three or CSEFEL invites everyone to work together in supporting the child's development.
When we are discussing the importance of play as a young child's primary vehicle for learning, I think it is important to bring up the huge body of research that has been done in major universities all over the world about early brain development, how young children learn, and how we can improve the way we support the learning process. I also find it useful to bring up the fact that there are continual advances in just about all aspects of most other fields, and periodically updating knowledge, skills and procedures is an expectation; in the field of early learning there are many, many teachers who are still teaching the way we did in 1985 (and many, many parents demanding it!) This begs the question "What's wrong with this picture?" I think part of the problem is that many of the teachers and caregivers currently working in our field, and most of the parents of the children we are caring for, went to preschool themselves in the 1980's and 1990's (or for older teachers, that's when they learned to teach) and they are simply knee-jerking to what they remember from their own childhoods. Regardless, it can also be helpful to describe in detail exactly what a young child learns when they engage in difference kinds of play, for example: "When children play with blocks, they develop meaningful skills in all areas of their development. Creating block structures develops an understanding of balance, symmetry, leverage, and ratios. Children learn about length, height, weight, area, size and function. They learn to predict cause-and-effect. They develop their creativity and sense of design. They utilize emergent reading and writing skills when they make props and signs for their block structures. They develop large and small muscle strength and skills, and strengthen their hand-eye coordination and depth perception. These are all precursors to reading, writing, math and science. Block play also offers a wealth of opportunities to refine social skills as children work together to create houses, zoos, towers, roads, castles, and so much more."Finally, I think it is useful to define the difference between "traditional" learning that focuses primarily on memorizing, reciting, and following directions without necessarily understanding what has been learned, and "developmentally appropriate, meaningful, foundational" learning which involves understanding what has been learned and prepares the child for the next level of learning.