If you're worried about a knee or ankle injury, then your kids should probably avoid soccer. If you're worried about your kid's brain, then they should NOT play full contact football (flag would be great). Studies are just coming out that show how sensitive a developing brain is from mini-concussions. They've also done IMPACT testing on high school football players that show their brain starts out at a certain level during their freshmen year, functions at a lower level by the end of football season, mostly recovers by the end of the year, and then that cycle continues all 4 years. This results in a slower functioning brain at graduation than what they had beginning of freshmen year. That's a huge problem.
I lucked out on this one by having daughters, but if I had a son it would definitely be a no go for full contact football at least until his freshmen year (with me crossing my fingers he loved baseball and basketball). Kids' brains just aren't made for that type of repetitive mini-trauma. Not worth it.