Building on our framework for integrating Indigenous Knowledge Systems into climate science curricula, this paper reports a participatory co-design process with Māori and Bhutanese communities. Climate science curricula in secondary schools have typically been developed by disciplinary experts and delivered through assessment regimes that privilege empirical measurement, leaving little space for the relational, place-based knowledge that Indigenous communities hold about changing environments. This study asked whether a participatory framework could produce a curriculum and, critically, an assessment approach that both communities and science educators would recognise as legitimate. Over fourteen months, co-design workshops were conducted with two iwi-affiliated communities in Aotearoa New Zealand and two village communities in eastern Bhutan, involving 58 community participants, 21 teachers and 6 climate scientists. Data comprised workshop records, artefacts, 34 follow-up interviews and pilot implementation in six classrooms. Thematic analysis identified four design principles that emerged in both settings: reciprocity in knowledge exchange, place as the organising unit of inquiry, intergenerational authority and the co-existence of explanatory frames. Outcomes include a shared assessment rubric that values both empirical and relational knowledge, structured around six criteria that assess observational rigour alongside relational understanding, community accountability and ethical conduct. Pilot use indicated that teachers could apply the rubric consistently and that students engaged with both knowledge forms. The paper proposes a participatory framework of five stages and discusses its transferability, its tensions and the conditions under which co-designed assessment can be sustained within national qualification systems.