One thing you could do is simply look on the Handbook of Nature Study blog for ideas for each season (go through her past posts). We own the actual Handbook of Nature Study book by Anna Botsford Comstock, so many times we simply look things up in there as we find them (or in field guides) to learn a bit more.
What specifically are you wanting? Are you looking for lessons, or for stories to read, or something else? If you give us examples we might be able to give you more helpful suggestions.
By the way, I have a 5th, 1st, K, and 3 younger, so I understand trying to keep them together as much as you can!