I found some online trees that show Zachariah as the son of Daniel Morris (1785-1871) and Mary Downey (1788-1835). Zachariah was born 5 Jan 1814 in Warren, KY and died 16 Apr 1891 in Greene, MO. It shows he was married to Keziah Diller. So perhaps this is the wrong Zachariah, or else perhaps Sarah was a second wife? Do you have source for their marriage?
The trees that show a Zachariah Morris born circa 1815 in Kentucky that died in 1864 in Madison, Arkansas show his spouse's name as Sarah (last name unknown). They don't show names for his parents. It appears that Simmons wasn't Sarah's maiden name, but rather she was first married to a Simmons before marrying Zachariah.
In 1850 census in Hilburn, Madison County Arkansas I find:
James Simmons age 21 (1829) b. KY Farmer
Sarah Simmons age 21 (1829) b. KY
Edy Simmons age 2 (1848) b. AR
On 1860 Hilburn, Madison Co AR census:
Zachariah Morris age 45 b. KY farmer
Sarah Morris age 30 b. KY
Eda Morris age 12 b. AR (Edy Simmons?)
Mary Morris age 5 b. AR
William Morris age 3 b. AR
Elizabeth Morris age 1 b. AR