There are several types of US Citizenship. Your children are described as statutory US citizens, i.e. they are U.S. Citizens by legislation written by the US Congress and signed into law by a US President.
Most legal scholars contend statutory US Citizens are Natural born citizens, but its debatable. For example, a statutory citizen born in Germany to US Citizen parents has the option to opt out of US Citizenship and choose to be recognized as a German.
The thought of a Natural born citizen opting out when he reaches the age of majority is contrary to very basis of the definition of a Natural born citizen. Even a naturalized citizen must renounce the citizenship of their home country before they can naturalize.
Current German law, and the law that existed when my children were born, requires at least one parent to be a German citizen for any children to be eligible for German citizenshp. Not sure what the law would be for children born in the UK, Spain, Portugal, Japan, Italy, etc.
I would not be happy if I found out, that because of my service to my Country, that my children were not considered Natural Born Citizens.