Are you suggesting it gives them an athletic advantage over the children of other American military personnel born and raised in Germany? I don’t think that is it at all.
When I was in the military in Okinawa, a large percentage of American soldiers who had married Asian women and had children with them chose to spend the rest of their military careers in Asia. They would do all of their long (36 month) tours in Asia. If they had to do an unaccompanied (12 month) tour, their family would remain in military housing at their last long tour post. Those that I personally knew did that primarily to avoid the racism they believed their children would encounter in the United States.
My Sergeant Major, for example, had four children with his Japanese wife. None had ever lived in the United States. They only had been there on holidays to visit his family. His oldest child, for example, had lived 7 years in Japan, 5 years in Okinawa (not part of Japan back then) and 3 years in the Philippines. The family remained in Japan while the Sergeant Major was stationed in Korea and in Okinawa while he was in Vietnam.
I suspect a lot of African-American soldiers today believe their children will experience less racism growing up if they do all of their tours in Europe (which mainly means Germany) rather than at large American Army installations, most of which are in the South. In part that is because the military bases and schools will have a significant number of mixed race students. I suspect a much lower percentage of Caucasian soldiers who marry Caucasian Europeans and have children with them choose to spend the rest of their military careers in Europe. To me, that is the most logical explanation of the “phenomenon” of the higher percentage of good soccer players among mixed race military children raised in Germany.