My son played HS football and baseball and is now playing D1 baseball. He's a terrific athlete and really wanted to play football, and he was good at it. As worried as I was about injury, how could I say no? The back story...
When he was in 7th grade his HS Freshman brother was diagnosed with cancer. Stage 4 lymphoma. It was bad. Real bad. I couldn't go on Google for months because I couldn't handle what I might learn. He spent 7 months in the hospital with my wife at his side enduring multiple surgeries and loads of toxins being injected into his body, with all the side effects. I pray no one else I know has to go through this. My ball playing 7th grader and I went to see them every night at the hospital after school and practice or cage sessions, where he did his homework. I did my best to keep things as normal as possible for him. After 7 months my son was released and spent the summer recovering and catching up on school. He did his entire freshman year of school in 10 weeks during the summer (...and ended up Valedictorian and going to Princeton). Needless to say, these things have a way of giving perspective and making the old saying "you only live once" pretty real.
That fall my younger son asked if he could play HS football the following year. Honestly, worrying about a knee injury or such seemed so trivial compared to pursuing a passion and making the most out of life. It still does 8 years later. I said ok, but you have to play Pop Warner in 8th grade so you can learn how to hit, be hit, tackle, etc (no way flag football comes close to preparing a kid). And that's what he did. And he excelled, had a most awesome experience, and is now preparing to be an officer in the Marines. I am so happy I did not get in the way of him becoming who he is today.
Oh, and to add to Paula's point, most of his college teammates played HS football or basketball or other sport.