Outdoor sketching is very enjoyable and the hope is that the child will continue to keep one the rest of her life. You'll find most Charlotte Mason moms keep their own because they like it. Buy some sketch pencils, colored pencils, brushes and watercolor paints and you're all set. Take your equipment on a nature walk and sketch specimens that you really see while you're out, if the weather does not cooperate collect things to bring home and sketch later. Do not worry about artistic talent, everyone will find out that it's much easier to sketch when you really look at what you're drawing.
Some people label their entries either in English or Latin or both.
Add some poetry or song lyrics for variety. Make sure you don't correct or look for spelling errors or any of that. When these books were used by students in Charlotte Mason's schools they originally called them a museum sketch book. That's why I like to make them using sketch paper. You could buy a blank sketch book or fill a three-ring binder with sketch paper, regular blank paper or lined paper.