IN EARLY CHILDHOOD EDUCATION, formative assessment should be ongoing and include multiple sources of evidence of children's existing knowledge. Children's understanding of mathematical concepts is highly diverse from a very early age, yet practical strategies to assess children's individual understanding are not always child centred and strengths based. This study explores the diverse ways in which 47 four-year-old children at three different early learning centres in metropolitan Melbourne demonstrated their number knowledge while they traced around wooden numerals, drawing and discussing values of quantity. Examples of children's representations of quantity are illustrated, accompanied by extracts of transcribed conversations. Findings demonstrate that this formative assessment strategy, through attuned prompts and skilled inquiry from the teacher, elicits children's complex understanding of number, located in the everyday experiences of their lives. Embedded in play-based activity, this assessment strategy is both engaging for children and highly productive for educators in documenting children's learning.