I listen to 夏休み子ども科学電話相談. It’s an NHK podcast, but I really like it because they have a group of scientists and professionals answer children’s questions about the ocean, bugs, space, and a bunch of other science-related topics. The Japanese is pretty simple because, well, of course, the professionals are making it easy for children to understand, but most of them speak really clearly anyways that it’s great practice for learners too.
I also appreciate that they’re pretty short. About 20 minutes an episode. One of my favorites is a doctor trying to explain to a 6 year old why he has to wear pants.