From Sally S. Harris, M.D., MPH

From the standpoint of injury risk, I wouldn't worry. Young children have a lower risk of injury than older children in contact sports like football, because they do not have the size and strength to cause forces great enough to cause serious injuries. A more important concern would be whether this activity and the associated competition is appropriate or necessary at this age. It is probably OK for a nine-year-old  child to participate if he or she enjoys it, and the emphasis is on participation and skill development rather than winning.