The source for these names is entirely secondary, and the birthdates look like pure guesses.  Based on the 1800 census, only three children were born by 1800 (two sons and one daughter), so either Nancy or Molly was born after the 1800 census.

There's a nine-year gap between Betsy and Abigail.  Were there really no children born during that period?

1.  LDS.  Family Search: Internet Genealogy Service:  IGI - International Genealogical Index (online at FamilySearch.org).

Orinda BRUCE; Birth: 26 Jul 1808  Betsy BRUCE; Birth: 27 Jul 1809 Abigail BRUCE; Birth: 10 Oct 1818 Samuel BRUCE; Birth: 02 Oct 1822 Loisa BRUCE; Birth: 10 Oct 1824 Lucinda BRUCE; Birth: 02 Jul 1827 Loren BRUCE; Birth: 30 Jul 1828 Edmund BRUCE; Birth: 08 Jul 1830 Location: Randolph Twp, Orange, Vermont Father: Samuel BRUCE; Mother: Hannah Source: General Index to Vital Records of Vermont, Early to 1870. Vermont. Secretary of State

I wonder if these are baptism or registration dates, rather than birthdates.

2.  1790 Census Index/Images (online at Ancestry.com, Image #3 of 4; extracted by Diana Gale Matthiesen):

1790

MA

Middlesex Co.

Marlborough

Roll M637_4

p. 374

Ln. 23L

Saml Bruce Junr

1-0-1-0-0

These data indicate:

Gender and Type	Age Class	Therefore Born	Individuals Inferred
free white males	16 or over	in or bef. 1774	= Samuel (b. 1771/2)
free white females	all ages	in or bef. 1790	= Molly (b. 1771)

3.  1800 Census Index/Images (online at Ancestry.com, Image #1 of 7 — list was roughly alphabetized; extracted by Diana Gale Matthiesen):

1800

NH

Cheshire Co.

Acworth

Roll 20

p. 755

Ln. 2

Saml Bruce

20010-10010-00

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	9 or under	1790-1800	= Abner (b. ca. 1793) = Jonas (b. ca. 1791)
1 male	26-44	1755-1774	= Samuel (b. 1771/2)
1 female	9 or under	1790-1800	= Molly (b. ca. 1795)
1 female	26-44	1755-1774	= Molly (b. 1771)

Where's Nancy?  Was she possibly born in 1800, not ca. 1797?  Listed next to Spencer BROWN.

4.  1810 Census Index/Images (online at Ancestry.com, Image #8 of 10 — indexed "Ruser"; extracted by Diana Gale Matthiesen):

1810

VT

Orange Co.

Randolph Twp.

Roll 64

p. 86/392

Ln. 16

Samuel Bruce

01010-22010

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	10-15	1794-1800	= Abner (b. ca. 1793)
1 male	26-44	1765-1784	= Samuel (b. 1771/2)
2 females	9 or under	1800-1810	= Betsy (b. 1809) = Orinda (b. 1808)
2 females	10-15	1794-1800	= Nancy (b. ca. 1797 or 1800?) = Molly (b. ca. 1795)
1 female	26-44	1765-1784	= Hannah (b. 1784)

Listed two lines from Jonathan AMIDAWN.

5.  1820 Census Index/Images (online at Ancestry.com):  not found, and I did a page-by-page check of Randolph Twp.; the only BRUCE in Randolph is Levi.

6.  1830 Census Index/Images (online at Ancestry.com):  not found, and I did a page-by-page check of Randolph Twp.; the only BRUCE in Randolph is Levi.

7.  1840 Census Index/Images (online at Ancestry.com, Image #3-4 of 30; extracted by Diana Gale Matthiesen):

1840

VT

Orange Co.

Randolph

Roll 543

pp. 107B-108A

Ln. 10

Samul Bruce

002 010 001 - 000 110 010

0400000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	10-14	1825-1830	= Edmund (b. 1830) = Loren (b. 1828)
1 male	20-29	1810-1820	= Samuel (b. 1819 or 1822)
1 male	60-69	1770-1780	= Samuel (b. 1771/2)
1 female	15-19	1820-1825	= Louisa (b. 1824)
1 female	20-29	1810-1820	= Abigail (b. 1818)
1 female	50-59	1780-1790	= Hannah (b. 1785/6)
4	persons employed in agriculture

The only other BRUCE in Randolph is Levi.

8.  1850 Census Every-Name-Index/Images (online at Ancestry.com, Image #60 of 64):  Randolph, Orange Co., VT, Roll M432_926, p. 242B, 478/478, enumerated 27 Sep 1850, official enumeration date 1 Jun 1850 (extracted by Diana Gale Matthiesen):

Samuel Bruce	78	M		Farming		Mass
Hannah   "	64	F				Conn
Edmund   "	19	M		"		Vt
Loren    "	22	M		"		"

Listed four households away:  Lonn E. BRUCE (æ 21, b. VT) is living with the MORRILL family.

9.  1860 Census Every-Name-Index/Images (online at Ancestry.com):  not found.

10.  Daniel S. Durrie.  1864.  A Genealogical History of the Holt Family in the United States, More Particularly, Descendants of Nicholas Holt of Newbury and Andover, Mass., 1634-1644…  J. Munsell, Albany.

p. 51

576. 5. KETURAH5 b. Windham Aug. 21, 1758; m. ___ Amidown.

11.  WorldConnect / Ancestry World Trees (online at RootsWeb.com/Ancestry.com).  Files there show Keturah HOLT born 31 Aug 1760.

12.  LDS.  Family Search: Internet Genealogy Service:  IGI - International Genealogical Index (online at FamilySearch.org).

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

14.  Peter Clegg.  Clegg Family Information (online at www.techmarketingink.com/clegg/family/).