Playing football is not the domain of adults only and kids can play it also but when kids are playing the game some precautions must be taken and they should also be taught to play the game as it is played on the international standard.
The first thing you should do is to make them know the rules with which they can become good players. They should also be suggested to play it in a field only and not in the lanes and by-lanes of the residential areas.
The thing that is of utmost importance before letting your kids allow playing football is the proper protection equipments. The two most important protection areas are the genital organs and the shin bone, after protecting these two you can allow your kids to play football. But they must have access to a doctor when need arises and you should make sure of it.