The risk is there whether a CI is involved or not: playing football, mma, roller derby, getting in a car, walking down the street ....
The only sports activity that CI manufacturers, surgeons, and researchers say a CI recipients should not do -- specifically for reasons relating to the cochlear implant -- is to dive below a depth of 25 meters (82 feet).
Any medical advice beyond that is valid, but unless it's based on some other medical complication involved with the specific patient, it's one knowledgable individual's risk assessment, what every one of us does when deciding whether or not to base jump, ski the black diamond trail on an icy day, or just plain get in the car and drive to work in the morning. Ask your doctor if there's a medical risk in your child playing football and the answer is most definitely yes. But that doesn't stop most people because they weigh the enjoyment of the sport in the balance against the high likelihood of injury. That's what people with CIs have to do, weigh in the position of that implant just under the skin at the side of the head along with likelihood of violence to that area among any other risk factors against the things that you can do to mitigate risks (helmets, training).
If my livelihood and or my measure of quality of life was tied up in deep sea diving, I likely wouldn't get a CI. But I'm not going to hold back my daughter from participation in ANY sports (other than deep sea diving) because of her CI, although I may hold her back because of general riskiness (I wouldn't want her to do football, ice hockey, kickboxing).