My daughter is eight years old and started riding several years ago. We have a really good coach, but are there any websites with picture/videos to show how the rider holds hands/body correctly. Horsemanship?
The barn that she rides at, does it offer different level lessons for everyone there? If so maybe she could ask the trainer is she can sit on the sides of a more advanced lesson and watch how they perform. Watching is usually the best way to learn when riding when you are not physically in the saddle. She will see how they are sitting and how they are being corrected by their trainer.
You could try videos such as The Visible Rider, or seeing if anyone you know has recorded any of the WEG or Olympic Dressage Competions, I know people who do and they are great to watch, especially if you could watch with someone who is more experienced and could point out the things that the riders are doing correctly.