This research focuses on young children's understandings about economic, social, and environmental sustainability. Participants were 43 children, aged from 3 to 7 years of age, recruited from two kindergartens in Croatia. Children's ideas were discussed through semi-structured interviews that focused on saving, spending, recycling, justice, inequality, diversity, gender stereotypes, interdependence, helping others, environmental pollution, and care of the environment. The interviews included closed and open-ended questions and photographs were used to illustrate the sustainability concepts discussed. The analyses of children's ideas about sustainability concepts and practices considered the complexity of children's ideas, using the SOLO learning outcomes framework. Older children were familiar with almost all key topics (e.g., use of money, recycling, diversity) but less familiar with the term justice, although they did have some understandings of its meaning. The youngest children had fewer ideas about sustainability. The findings indicate that kindergartens and preschools can provide foundations for lifelong learning about sustainability and important implications are that teachers can promote children's understandings through enriching pedagogical activities.