PYONGYANG, May 6 (Reuters) - Foreign journalists invited to cover North Korea's first ruling party congress in 36 years were treated on Thursday to song and dance performances by schoolchildren professing their love for leader Kim Jong Un. Kim is expected to use the congress starting on Friday to declare North Korea a nuclear weapons state and formally adopt his "Byongjin" policy to pursue economic development and nuclear capability at the same time. The presence of foreign reporters offers the world a rare view from inside the isolated country, but their movements are closely managed.