Sorry I cannot help.  I looked up all my Mixson's last night, but did not 
locate yours.  I am from Hampton originally,  An ancestor of mine, David 
Rivers, married Philadelphia Mixson.  I haven't had much luck with her 
parents either.  I looked in the 1850 census.  Your William would be 8, but I 
cannot find anyone with an 8 year old   
son/  There is a William Mixson with wife Cynthia and a one year old son.  
Were your Mixsons in Prince William Parish or in St. Luke's or St. Peter's?  
Mine are in Prince William in what we call Pondtown, an area between 
Crocketville and Brunson.
Keep in touch.  Heyward Sauls (PO [email protected])