My ten year old son has played tackle football for the past three years. I used to talk about it all the time, but lately I keep it to myself because I don't want to be accused of abusing my son or something. Fact is, he just loves it. Football is the center of his universe, and oddly, not because of me. I didn't play and we're not a 'football family' or anything. He just discovered it himself and loves playing. I admit unlike most parents I go and sit and watch every single practice because I want to keep an eye on the coaches and make sure they aren't pushing the kids in the wrong direction (yes, I'm that dad) but I just can't not let him play. My sport was Tae Kwon Do, and over the years I've broken one wrist, sprained the other, dislocated my jaw and suffered a fairly significant concussion. I saw one fellow student (and high-level competitor) have his arm shattered in three places blocking a kick during practice which now has a big ol' metal plate in it.
That's sports. If you are active and you put your heart and soul into it, it's going to take a toll. And if you compete at the highest level, your body will fall apart much sooner than the rest of us.
I don't expect that my son will play professional football. And at this level, I think keeping him active and engaged and letting do something he truly enjoys balances out the risks.
Anyway, just some input from someone who does let their kid play.