I have a KEWLEY family also, who were from Douglas and members went over to Liverpool at about the same time as your Thomas. I did a search for Thomas and found the family on the 1841, 1851 and 1861 census's in Liverpool but they seem to disappear after that. The eldest child that appeared in the census was Elizabeth age 3 in 1841. I also searched the FreeBMD for a marriage, but none showed up. They might have married just before English civil registration began in Sept 1837.

You didn't mention a surname for Thomas's wife Elizabeth. If you don't know it, you could obtain a birth certificate for your g-g-grandfather William. I found a reference that seems a good fit: Births Dec 1845 Quarter, William KEWLEY Liverpool District 20 518. If we knew her name, we could look again at the very early civil registrations under her name just in case Thomas's surname was mistranscribed.

The 1861 census had a daughter Helen 17 and your William 15, neither of whom were with the family in 1851. I couldn't find them anywhere in the English or IOM 1851 census other than there was an Ellen KEWLEY niece age 7 b Liverpool with a Charles & Sarah ASHTON. That might be a different person. However, I would think they'd be there somewhere and perhaps with relatives (which could be a very useful clue). If the surname was mis-transcribed, it becomes much harder to find. Ancestry came up with 449 people with the first name of William born in 1846 in Liverpool. That's more than 1 William per day!

Based on ages, William KEWLEY was born about 1812 to 1814. The only Thomas Kewley baptized in Douglas during this period was on 29 Jan 1814 son of Thomas KEWLEY & Isabella CORRAN at St. George, Douglas. There's also one on 1 Jan 1815 son of John KEWLEY & Margaret CALLOW at St. Mathews Douglas