I agree that Benjamin and Mary don't appear to be the parents of John H. Impson. But, John Impson and Temperance Harris are a likely bet. I think that the Tempy Simpson [which probably should be Impson] was Temperance Harris Impson. I have not been able to see the actual 1850 census, but I think that the census taker heard "Simpson" instead of "Impson" or he actually wrote "Impson" and whoever put the info on the Internet mistook "Impson" for "Simpson". And, John is a likely name for John Henry's father. Where did you find the names of John Impson and Temperance Harris?