I just received the following information from Linda Boorom:
Frances was indeed a Cox and married before, but not to a Canterbury. The marriage record & the 1850 census recording her children as Canterbury's were just to throw us off track :-)
She married as "Franky Cox" on Oct 25 1827 in Cape Girardeau Co., MO to Don L. Van Amburgh AKA VANDENBURGH.
They had 3 children:
1) Sarah married Allan J. Waugh May 29 1845 Cape G. In 1860 Frances Canterbury is residing with her. Sarah appears to be a widow age 31 with 4 children, Don age 12, Franklin age 8, John age 6 & Frances age 3.
2) John Van Amburgh age 30 in 1860 Hubble Twp with wife Amanda, children James age 4 & John age 1
3) Benjamin Van Amburgh AKA VANDENBERG married Nov 24 1857 none other than Mary F. Ford d/o Chesley Ford & Rebecca Canterbury! In 1860 he is living next door to his brother John with 1 child, Thompson C. age 1.