Sorry I don't have any book suggestions for you. But when I was a child I was very interested in gardening for two reasons: I would go with my grandmother and "help" her as she gardened. Sh'ed let me dig planting holes for her, pick flowers, pull weeds, plant bulbs, things like that. I appreciated the time I spent with her. And: 2. My parents encouraged me to plant a "salad garden" of various lettuces and radishes. Tomatoes would have taken too long. These plants came up pretty quick, then I'd pull them up and make a nice salad for my family. With a home-made dressing of vinegar, oil, water, salt and pepper to taste. These days you can buy "good seasons" packages of dressing mix.

I have a book named Plants, a Golden Photo Guide from St. Martin's Press that might be perfect for a 10 year old. Despite its small size it contains loads of information and pictures. I'm not trading this year because I'm getting ready to sell my house, but drop me your address and we'll get your girl fixed up.