How about this: a free copy of the ultimate guide to nature study!
Handbook of Nature Study
by Anna Botsford Comstock. (1939).
This is the definitive textbook on teaching nature study to children. You can find it for free on Google Play Books because it is out of copyright, but you will see that it is still for sale on Amazon and many other sources. It’s a huge book and has over 900 pages. If this was the only resource available to you, you’d still be able to teach nature study to your children for years with the material in this book.
Part one discusses the teaching of nature study, with information and ideas as relevant today as they were when they were written. The next parts are divided into lessons and cover animals, plants, and earth and sky. You can go in order, or pick and choose which lessons you’d like to use. There are over two hundred lessons!
You’ll find observation guidelines, review questions, and writing assignments included within. This is an incredibly complete reference and is indispensable for anyone who is thinking of teaching nature study to children.
There is a Librivox recording of this book that you can find HERE
Some more ideas:
If you’d like to see some truly gorgeous old-fashioned children’s books, please visit Children’s Books Online: The Rossetta Project. They have scanned in some beautiful older books so you’ll get to see the original artwork in its original color. You can’t download these, but you can read them online.