You'd have to find William and Mary's marriage which is likely to have been in Guernsey in the late 1830s to get Mary's surname. Do you have access to Ancestry? If you search on Ancestry's 1841 Census for Channel Islands for William Callister or Callester you'll find William, Mary and a son called Thomas.

Bordage Street, St. Peter Port, Guernsey:-

William Callister, 35, Mariner, E
Mary Callister, 22, Y (Guernsey born)
Thomas Callister, 1, Y (Guernsey born)