Children's conceptual knowledge of writing words and numbers is an important aspect of their cognitive development. Children use notations as representations that have a communicative value and begin to learn about formal differences between writing words and writing numbers at an early age before the onset of formal schooling. The aim of the present study was to examine preschool children's conceptual knowledge of writing words and numbers in an ecologically valid task with communicative value. One hundred and twenty Greek-speaking preschool children between the ages of 3 and 6 years old were assessed on the identity card task, which examined their production of notations for words and numbers during their effort to communicate personal information to others. The results demonstrated a developmental trend in the type of notation children produced with the younger children providing more "ambiguous" notations, for both tasks implying writing words and numbers, while as they got older, they provided more "writing-like" notations for the tasks implying writing words and more "number-like" notations for the tasks implying writing numbers. Understanding when and how children differentiate their symbolic representations for words and numbers can inform both theory and practice by expanding our understanding of whether certain constraints characterize the developmental course of a specific notational system. The findings of the present study could be incorporated in educational practice and enhance children's emerging literacy and numeracy skills.