Football is a violent and dangerous sport. You can't sugar coat it... If your kid keeps playing up through high school he'll get hurt at some point.
However, at the age of 7 - 9 they are just as safe playing tackle football as they are playing soccer. IMHO that is the best age for tackle football. They learn the game, learn how to tackle, learn how much work is required to keep playing.
I hate when people say "Flag until their teenage years"... You take that teenager and put him up against a kid whos played tackle for the past 6 years and he's going to get destroyed.
Football @ 14-18 years olds... Not for everybody.
Football @ 11 - 14 years old... Kids start getting hurt. Kids give up. Parents get fed up.
Football @ 7 - 11 years old... The best football. The fun football.