Learning to write is a complex process involving linguistic, cognitive, and socio-emotional factors. From a developmental perspective, little research has explored the content of young children's writings in terms of specific cognitive skills such as Theory of Mind (TOM). This study explores how young children's writing may foster representations of inner life, their own and others, through the characters created and through an awareness of the reader's perspective. A total of 204 written compositions were gathered throughout one school year from 24 6-to-7-year-olds. The results indicate that while tackling the complexity of word and sentence composition, significant cognitive resources are allocated to the content of the stories, the characters' experiences but also the reader's understanding of the plots created. The findings offer important implications for supporting children's TOM through early fiction writing and highlight the importance of naturalistic data in uncovering young children's growing insights about others' thoughts and emotions.