Hi Sidney
I had a quick look on the IGI. There were only 116 entries for that name, the most common spellings being McJennet/McJanet. Most of the entries were from Ayrshire so the origins of the name must be from there. As you found, I couldn't see any entries that fitted for Robert or any Williams with Robert as father.
Do you have any information on Robert's wife that you could research?
I tried the official Scottish site, Scotlands People using wildcards to pick up on the different spelling variants, m*cj*net*. Some entries came up for both Roberts and Williams. You won't know if they are the right ones until you view.
Did the family remain in Scotland or did they immigrate? If they left Scotland after 1841/51, you would probably find it helpful to find census info for those census years. Scotlands People have stated that they intend to make those census indexes available on line in the first quarter of 2006 (fingers crossed!).
If they remained in Scotland, have you tried looking for marriage/deaths certs (post 1855 for Deaths)?
Regards.
Monica