Have you checked the 1860 census and verified that these people were free prior to 1870? Since the record you have is in 1870 and the people are fairly young, it is more than likely your family was freed at the end of the Civil War, in 1865. There would have been no emancipation records for after the War. There were several Knight families in the 1700's in Halifax County which were freed, but you don't mention if you are looking for someone that far back.