It's a real stretch, biologically, for Susanna to be the mother of these two children.  Are they grandchildren?
  9.  Margaret JOHNSTONBAUGH, b. 1829/30
10.  John JOHNSTONBAUGH, b. 1831/2

1.  Marriage Record: 
 

2.  Census Index:  U.S. Selected Counties, 1800 (Broderbund CD-312):  not found — nor any other JOHNSTONBAUGH or variation beginning JOHN-/JON-, only SCHANTZENBACH, specifically George SCHANTZENBACH.

3.  1810 Census Index and Digital Images (online at Ancestry.com; Image #1 of 3; extracted by Diana Gale Matthiesen):

1810

PA

Centre Co.

Walker Twp.

p. 164

Jacob Johnstonbaugh

10100-11100-00

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	9 or under	1800-1810	= George (b. 1809)
1 male	16-25	1784-1794	= Jacob
1 female	9 or under	1800-1810	= Daughter A
1 female	10-15	1794-1800	= ?
1 female	16-25	1784-1794	= Susanna (b. 1783)

4.  1820 Census Index and Digital Images (online at Ancestry.com; Image #1 of 9; extracted by Diana Gale Matthiesen):

1820

PA

Centre Co.

Ferguson Twp.

p. 236

Jacob Johnstonbough

210010-31010-0-200

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	9 or under	1810-1820	= Thomas (b. 1819/20) = Jacob (b. 1815)
1 male	10-15	1804-1810	= George (b. 1809)
1 male	26-44	1775-1794	= Jacob
3 females	9 or under	1810-1820	= Lydia (b. 1819/20) = Mary (b. 1813) = Sarah (b. 1811)
1 female	10-15	1804-1810	= Daughter A
1 female	26-44	1775-1794	= Susanna (b. 1783)
2	persons engaged in agriculture

5.  1830 Census Index and Digital Images (online at Ancestry.com; Image #19 of 22; extracted by Diana Gale Matthiesen):

1830

PA

Centre Co.

Ferguson Twp.

p. 408B/409

Jacob Johnsonbaugh

002 010 100 - 020 200 100

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	10-14	1815-1820	= Thomas (b. 1819/20) = Jacob (b. 1815)
1 male	20-29	1800-1810	= George (b. 1809)
1 male	40-49	1780-1790	= Jacob
2 females	5-9	1820-1825	= Susannah (b. 1822) = Lydia (b. 1819/20)
2 females	15-19	1810-1815	= Mary (b. 1813) = Sarah (b. 1811)
1 female	40-49	1780-1790	= Susanna (b. 1783)

6.  1840 Census Index and Digital Images (online at Ancestry.com; Image #15-16 of 34; extracted by Diana Gale Matthiesen):

1840

PA

Centre Co.

Harris Twp.

pp. 128B-129A

Thomas Johnstonbaugh

010 010 - 000 020 010

0100000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	5-9	1830-1835	= John (b. 1832)
1 male	20-29	1810-1820	= Thomas (b. 1819/20)
2 females	20-29	1810-1820	= Mary (b. 1813) = Sarah (b. 1811)
1 female	50-59	1780-1790	= Susanna (b. 1783)
1	person engaged in agriculture

Jacob was not found as head of household.  The numerical data indicate that Widow Susanna and some of her children are living with her unmarried son, Thomas JOHNSTONBAUGH.  The youngest daughter, Susannah (b. 1822), is still unmarried, so why is she not at home?  Is she living with one of her married siblings?  Or hired out as a servant?  Listed one line above Thomas's brother, Jacob JOHNSTONBAUGH, who is listed seven lines above their brother, George Washington JOHNSTONBAUGH.

7.  1850 Census Index/Microfilm (online at GenealogyLibrary.com):  Harris Twp., Centre Co., PA, p. 108A, 1350/1372, enumerated 4? Nov 1850 (extracted by Diana Gale Matthiesen):

Susanah Johnsonbach	67	F		6000
John         "	18	M	Farmer
Susanah      "	10	F

The ten-year-old Susanah is too young to be widow Susanna's child, and it's a real stretch for John to be her son.  This enumerator left the birth location blank for everyone on the page, except the first person on the page who was born in PA, with the implication that everyone on the page was born in PA.

8.  Descendants of Jacob Johnstonbaugh by Cindy L. Johnston.  Ms. Johnston appears to be quoting here from a WFT file she imported:

Jacob left for Ohio sometime in the 1830's with $1500 to buy lamd there.  When he never returned or let his family know where he was, George W. started a search.  He was traced as far as Pittsburgh and from there he could not be traced any more.  They concluded that he was probably robbed and killed for the money he was carrying.  His wife was in desparate straits because she could not sell anything until she proved he was dead.  Not being able to do this she did the next best thing, she divorced him.  We can not understand why Jacob wanted to go to Ohio for more land when he had plenty in Centre Co.  Between he and his wife they gave each of their children 50 acres of land.  By the way Jacob's name was Schantzenbach.

9.  William H. Welch.  1978.  "Information Wanted." The Pennsylvania Genealogical Magazine XXX(3): 212-214.  On p. 213:

JOHNSTONBAUGH. (Schantzenbach Sentzenbach).  Wish ancestry birth death data of Jacob Johnstonbaugh, born ca. 1784; died before 1840 census of Harris Township, Centre County, Pennsylvania.  Confirmed at Evangelical Lutheran Church, Aaronsburg, Haines Township, Centre County in 1801.  What relationship to George and Jacob Shantzenbach, Upper Saucon Township, Lehigh County, Pennsylvania?  He married Susanna Hubler. Mrs. William H. Welch, Box 83, R.D. 2, Mill Hall, Pennsylvania 17751.

10.  Broderbund.  World Family Trees.  Vol. 6, Pedigree #2310.

11.  WorldConnect / Ancestry World Trees (online at RootsWeb.com/Ancestry.com).