We live in an increasingly worrying future, where researchers are looking at how we can create education for a sustainable world. In this study we investigate what such education could look like, especially for young students, reinforcing hope and an ability to act and come up with ideas of sustainable solutions with no obvious 'good' solution. The analytical tool suggested in this paper addresses the challenge of promoting and illuminating ethical reasoning in early childhood education for sustainability and mathematics. The teaching of ethics should use children's experiences as a starting point rather than theories of ethics. Using the topics of sharing as an example, and the results from a series of empirical studies, we illustrate how ethical arguments can function as backing for mathematical reasoning, and how mathematical arguments can play a similar role in ethical reasoning. We argue that the tool could assist in teaching where mathematics and education for sustainable development merge through ethical reasoning but also as an analytical tool for research.