My favorite children’s book about nutrition is by Dr. William Sears, called Eat Healthy, Feel Great. All my bigger children love it; including my 12-year-old. I would read it as a family, since it’s more on the 7 or 8 year old level, and my son might be a bit offended if I assigned it to him alone. It’s excellent though, and has a lot of good info. There is a series about healthy eating/herbalism called Herb Fairies that is superb. I haven’t used it myself since we don’t do fairies, but the series is so good, I’m almost tempted!