The idea of a class debate is a great one! I think that it would have to be set up obviously in a different way working with younger students, but I think it would give them great practice in reading informational text and pulling out pieces of evidence from which to make their argument. Perhaps having pre-selected texts for them to work from? Or if the Green Ninja cartoon is something they really enjoy -- you could task your students with creating a cartoon / mini-video that explains their stance on geoengineering.
Climate change science can definitely feel overwhelming, and I agree with you that often times when talking about communities that are distant from their own, or money in the billions, you can lose the authenticity and connection to students. I think it is really important in focusing in on conservation behaviors and messages that relate directly to our students and highlighting the changes they can make in their own lifestyles.