This is a concern shared by many parents. Injuries do occur in any sports activity at any age. Young children seem especially vulnerable. And the number of kids participating in all kinds of sports is steadily increasing. So the risk of injury is also increasing.
Head and neck injuries are rare in this age group. But the National Pediatric Trauma Registry does report 1.5 per cent of all pediatric trauma patients are seen for a cervical spine (neck) injury. And 20 per cent of those injuries in children aged eight years and older were sports related. Other causes included car accidents and falls.
The rising number of children engaged in sports activities suggests a need for specific guidelines in managing emergency situations and injuries. Researchers are just beginning to take a look at some of the issues.
Size difference can certainly make a difference. As you suggest, children come in all sizes and shapes. Their body sizes develop at different rates. The growth curve is variable within a small age range (e.g., eight to 10 or 10 to 12). Add the weight of football equipment such as helmet and shoulder pads and the picture can change.
It would be a good idea to bring your concerns to the coaches and organizers of the program. Most likely they have many safety measures built into the program. If not, your questions, concerns, and participation in developing appropriate guidelines would be very timely.