I truly enjoy a wonderful notebook. Like Richele, I'm a paper lover and Moleskine are a favorite of mine. And like DawnB, we've used Waldorf notebooks, and I am fond of them. What I especially like about the Waldorf notebooks for copywork is that they are thin and don't have many pages. It's doesn't take long to fill them and have a feeling of accomplishment.
However, sometimes (many times, most of the time?) I simply grab a stash of unbound paper and a clipboard and use that for nature journals, artwork, copywork, etc. The unbound paper might be drawing or watercolor paper, but often it's just the paper I use in my printer.
I noticed I had a habit of "waiting" for the right supplies in order to begin, and that bothered me. Also, I think my kids were less intimidated by a plain piece of paper when they started their nature journals. And, it's easier to handle one piece of paper than a notebook while you are drawing or painting, and it's not frustrating if you don't like your results and want to start over. Even for myself, I often prefer the plain paper in the field and then I paste it into a nice notebook after I get home, make a small homemade book with the drawings, paste them on notecards for stationary. I also rotate drawings in a set of frames we have on display in our living room, and sometimes I simply pin the most recent drawings on the kitchen bulletin board.
For the kids' nature journal, for years I put their drawing in a photo album. The album accomodates any size of drawing, allows us to add things like text, ticket stubs, maps, etc. and it's so easy to put pages together - no glue or coordination required (I often lack both!). Now that they are older, they might enjoy a nice notebook like I use for my own drawings...I'll have to ask them.
While I truly have a passion for paper and supplies, usually we prefer the unbound sheets of paper. Whatever gets you and your kids drawing and doing copywork is the best material to use.