Although kindergarten children are the group of citizens who will face the consequences of today’s environmental challenges, research on their knowledge about environmental sustainability is limited. In June 2019, we interviewed 56 children (five to six years old) from eight kindergartens in Norway and asked kindergarten staff and the children’s parents to fill out a short questionnaire. The aim of the study was to investigate children’s knowledge of how our actions affect the natural environment and children’s self-declared sense of belonging to nature. Our results show that upon completing kindergarten, many children had gained an early understanding of environmental sustainability. Garbage disposal, deforestation, and air pollution from vehicles were the environmental issues children were most aware. We detected a positive association between time spent in nature with parents and children’s knowledge; this emphasizes the importance of children spending time in nature with their parents.