child prodigy       
noun [ C ] uk/ˌtʃaɪld ˈprɒd.ɪ.dʒi/ us/ˌtʃaɪld ˈprɑː.də.dʒi/
 神童,天才儿童
a young child who has very great ability in something
  A child prodigy, he made his first professional tour as a pianist at the age of six. 他是个神童,6岁的时候就作为钢琴演奏家进行了首次职业巡回演出。