According to information published by the Ipswich Historical Society: (Genealogies of William Fellows etc in: Publications of the Ipswich Historical Society, Issues 16-20, 1909, Salem, MA)
Williams Fellows son of Isaac and Joanna, is the William Fellows who married Sally P. Haskell of Gloucester. in 1823, she died in 1837.
Vital Records of Ipswich Massachusetts to the End of the Year 1849, The Essex Institute, Salem, Massachusetts, 1910
Marriage: Fellows, William, and Sally P. Haskell of Gloucester, Nov. 13, 1823. (p. 162) Deaths: Fellows, Sally P., w. William, June 30, 1837. [a. 45 y. PR3] (p. 533)
Their children listed in the Ipswich VR are baptisms, not births are: Isaac, James Haskell, David Brainerd, and Winthrop
I don't know if he had a prior wife and/or if he remarried after Sally Haskell died, but his probate record indicates more children than those baptized at Ipswich..