Environmental education is becoming a growing area of interest in early childhood education. In France, more time is given to environmental education activities in the preschool curriculum. Yet, there is a risk of narrowing preschool environmental education only to carry out pro-environmental behaviours, without giving pupils the opportunity to connect these practices to the scientific knowledge behind. Drawing on the concept of socialisation, the article explores how the interplay of the social and geographical backgrounds, as well as implementation by the family of biowaste management and how this influences 5-year-old children's understanding of organic matter decomposition. We used semi-structured interviews with parents and young children to determine details of parental composting execution on the one hand, and children's knowledge and misconceptions about the organic matter cycle in a composter on the other. We collected data from samples in neighbourhoods with different socioeconomic characteristics in two different French cities. Results show that young children are capable of developing an early understanding of biowaste decomposition in the context of composting. This study also shows that all the variables involved in the child's immediate environment must be analysed to understand how their ideas are constructed, and how their observations are interpreted.