Although parents and preschool play important roles in developing children's behavior and attitudes, little is known about their influences on children's learning of environmental, social and economic aspects of sustainability. This study investigated the influences of home- and preschool-related practices and factors on children's declarative and functional knowledge of sustainability issues, and the extent to which eco-certified preschools promote beneficial practices. 'Eco-certified preschools' refers to schools that explicitly work with education for sustainability. Children (n=53), aged five to six years, and the directors (n=7) at six eco-certified and six non-eco-certified preschools were interviewed, while guardians (n=89) and teachers (n=74) filled out questionnaires. Children's responses were categorized and classified using SOLO Taxonomy. Multivariate analyses were performed in SIMCA P + 14. The findings indicate a positive relationship between children's declarative and functional knowledge of sustainability issues and the involvement of teachers and guardians in sustainability-related discussions and activities. Teachers' verbal interaction with children about sustainability issues, and the perceived high value of these issues among teachers and directors seem to be more beneficial for children's declarative knowledge than their functional knowledge. No statistically significant differences between eco- and non-eco-certified preschools in terms of children's declarative and functional knowledge were found.