A slightly different set of children can by hypothesized based on the census data.  The above scenario works only if Daughter B was born in 1810.

1.  Census Index:  U.S. Selected Counties, 1800 (Broderbund CD-312):  not found.

2.  Family Quest Archives:  Pennsylvania Federal Census 1810:  Washington, Wayne, and York Counties.  Heritage Quest CD M252-53 (extracted by Diana Gale Matthiesen):

1810

PA

Washington Co.

Finley

Image #101

p. 110 (penciled)

p. 100 (stamped)

Philip Dennis

10100-20010-00

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	9 or under	1800-1810	= Son A
1 male	16-25	1784-1794	= Philip (b. 1780)
2 females	9 or under	1800-1810	= Daughter B (b. 1810) = Daughter A
1 female	26-44	1765-1784	= Elizabeth (b. 1785)

Philip and Elizabeth are in each other's age class.  I don't doubt that this is our couple, so it appears to be census-taker error.

3.  An Index and Transcription of the 1820 Census of Knox Co., OH (extracted by Diana Gale Matthiesen): 

1820

OH

Knox Co.

Jackson Twp.

p. 19A

Philip Dennis

310010-11001-0100

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
3 males	9 or under	1810-1820	= Son C = Son B = Isaac (b. 1811/2)
1 male	10-15	1804-1810	= Son A
1 male	26-44	1775-1794	= Philip (b. 1780)
1 female	9 or under	1810-1820	= Daughter C
1 female	10-15	1804-1810	= Daughter B (b. 1810)
1 female	45 or over	in or bef. 1775	= Elizabeth (b. 1785?)
1	number engaged in agriculture

Has Daughter A died?  And here is Elizabeth, in a higher age class, again.  Was she really born in 1785?  Or is this more census-taker error?

4.  Family Quest Archives:  Ohio Federal Census 1830:  Jackson, Jefferson, Knox, Lawrence, and Licking Counties.  Heritage Quest CD M19-134: (extracted by Diana Gale Matthiesen).

1830

OH

Knox Co.

Jackson

p. 212A

Dennis Philip

002 110 100 - 011 100 100

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	10-14	1815-1820	= Son C = Son B
1 male	15-19	1810-1815	= Isaac (b. 1811/2)
1 male	20-29	1800-1810	= Son A
1 male	40-49	1780-1790	= Philip (b. 1780)
1 female	5-9	1820-1825	= Daughter D
1 female	10-14	1815-1820	= Daughter C
1 female	15-19	1810-1815	= Daughter B (b. 1810)
1 female	40-49	1780-1790	= Elizabeth (b. 1785)

5.  Census Index:  U.S. Selected Counties, 1840 (Broderbund CD-316):  as expected, Philip was not found, but neither was Elizabeth.  She must be either remarried or living with one of her children.

6.  LDS.  Family Search: Internet Genealogy Service: Pedigree Resource File (index to CDs online at FamilySearch.org).

Compact Disc #12 Phillip Dennis Birth: 10 Aug 1780; Place: Washington, Pennsylvania Death: 15 Aug 1835 Burial: ___________; Place: Bladensburg Cem, Jackson, Knox, Ohio Father: Michael Dennis; Mother: Rosannah Wolf Spouse: Elizabeth Horn Marriage: 1805

7.  LDS.  Family Search: Internet Genealogy Service: International Genealogical Index (IGI).

Isaac DENNIS Birth: 1812, Washington, Pennsylvania Father: Philip DENNIS; Mother: Elizabeth HORN Source: LDS, official temple record for deceased individual...

8.  LDS.  Family Search: Internet Genealogy Service:  AF - Ancestral File (online at FamilySearch.org).

9.  WorldConnect (online at RootsWeb.com).