— born in MO:
5.  Melvina JACKSON, b. 1833/4; d. 21 Feb 1901, DeSoto, Jefferson Co., MO
6.  Emily D. JACKSON, b. 1836/7
7.  George Washington JACKSON, b. 6 Jan 183}; d. 3 Jan 1921, Denver, CO
8.  Lucinda E. JACKSON, b. 1840/1

William's and James's birthyears can't both be correct; as the censuses give James a birthyear between 1831 and 1835, it appears James is the one not born in 1830.

1.  Byron Sistler & Barbara Sistler.  1987.  Early East Tennessee Marriages.  Byron Sistler & Assoc., Inc., Nashville, TN:  no antebellum marriage records for Morgan County have survived.

2.  1830 Census Index/Images (online at Ancestry.com; Image #19 of 30; extracted by Diana Gale Matthiesen):

1830

TN

Morgan Co.

Roll M19_180

p. 97

Ln. 10

Benjamin Jackson

000 001 - 100 010

no slaves

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	30-39	1790-1800	= Benjamin
1 female	4 or under	1825-1830	= Martha (b. 1828)
1 female	20-29	1800-1810	= Wife

There's only one other Benjamin JACKSON indexed in the 1830 census of Tennessee, and he's too old and has too many children to be our subject.

3.  1840 Census Index/Images (online at Ancestry.com, Image #9-10 of 14; extracted by Diana Gale Matthiesen):

1840

MO

Washington Co.

Belview Twp.

Roll M704_133

pp. 202B-203A

Ln. 14

Ben Jackson

130 000 100 - 111 001

0100000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	4 or under	1835-1840	= George (b. 1839)
3 males	5-9	1830-1835	= Son A = James (b. ca. 1831/2) = William (b. 1830)
1 male	40-49	1790-1800	= Benjamin
1 female	4 or under	1835-1840	= Emily (b. 1836/7)
1 female	5-9	1830-1835	= Melvina (b. 1833/4)
1 female	10-14	1825-1830	= Martha (b. 1828)
1 female	30-39	1800-1810	= Wife
1	person employed in agriculture

4.  1850 Census Index/Images (online at Ancestry.com; Image #135 of 153):  District No. 42, Jefferson Co., MO, p. 474A, 999/999, enumerated 23 Oct 1850, official enumeration date 1 Jun 1850 (extracted by Diana Gale Matthiesen):¤

William Jackson	20	M		Farmer	150	Tenn
Martha     "	21	F				Tenn
James      "	18	M				Tenn
Melvina    "	16	F				Mo
Emily      "	13	F				Mo
George     "	11	M				Mo
Lucinda    "	9	F				Mo
Lucinda Evans	66	F				Va

5.  Messages posted to the JACKSON Surname Board (online at Ancestry.com), including one by Mary Miller giving this information:

According to Probate Inventory records of Benjamin JACKSON, dated October 25, 1847, found in Circuit Court records, Potasi, Washington County, MO, Benjamin and his wife died prior to that date.  James SEABOURN(e) was the Administrator of his probate, (the LDS, IGI shows Lucinda Evans as James SEABOURN's mother.)  The Probate names the following children as children of Benjamin JACKSON:  Martha Jane, William Andrew, James Monroe, Melvina, Emily (D,) George and Lucinda (E) JACKSON.  In the probate, they were residing in Washington County with "Mrs. Evans."  In the probate, there were receipts for 2 coffins, (Benjamin and his wife must have died at the same time,) expenses for the burial from Caledonia, dated November 24, 1847.  The receipt for medicine, dated December 27, 1847, reads, "for medicine and other necessaries during the last illness of Benj. JACKSON and wife and for shrouding for Mrs. JACKSON."  The burial site of Benjamin and Mrs. JACKSON are unknown at this time.  James M. SEABOURN (spelled Saborne) became the guardian of James age 19, Emily age 15, George W. and Lucinda E. Jackson, on July 21, 1852.