I'm not normally a Berenstain Bears fan (understatement), but the Berenstain Bears Big Book of Science and Nature came highly recommended from friends, and I believe Sonlight uses it as well. My 5 year old absolutely loved it, and had me read through it with him at least three times this past year. It would make a nice supplement. I bought ours used, on these boards I think.
If you want more of a Charlotte Mason approach, with lovely stories about nature, I would recommend Among the Meadow People, Among the Forest People et al. We have enjoyed them very much.