I’m in the midst of researching youth fitness issues, as I prepare to teach cross-country for the first time this year. My wife suggested Fit Kids! by Dr. Ken Cooper. I’ve been familiar with Dr. Cooper’s work for many years; he has a phenomenal (and massive) health clinic in Dallas. He actually invented modern aerobics. Nobody has more breadth and depth of knowledge about general fitness, in my opinion.
All that to say that this book is an excellent resource about kids’ exercise, diet, stress, and sports. He is not extreme in any way, and he bases his opinions on research, not mere experience. It’s a bit old (last updated in 1999), but I didn’t find anything that seemed dated in any way. You can buy it for under $2 from Amazon or $1 on Half.com (my favorite), so it’ll be delivered to your door for about $5. My wife and I highly recommend it, whether you are a youth sports coach, PE teacher, or a parent.