Both William and Dorcas were old enough to have had an earlier marriage, which means the eldest daughter may have been from a prior marriage, which may explain why she is not as well known as the other children.  It may also be that she died young.

1a.  Jordan R. Dodd, ed.  1993.  Kentucky Marriages:  Early to 1850.  Liahona Research, Orem, UT (Broderbund CD-229):

Robards, William

Maxwell, Dorcas

14 Feb 1809

Fayette Co.

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

William R. ROBARDS Spouse: Dorcas MAXWELL Marriage: 10 Jun 1809, Lexington, Fayette Co., KY Source: none given

Sarah ROBARDS Birth: 18 Oct 1811, Lexington, Fayette, Kentucky Father: William R. ROBARDS; Mother: Dorcas MAXWELL Source: none given

Zarilda ROBARDS Birth: 23 Aug 1813, Lexington, Fayette, Kentucky Father: William R. ROBARDS; Mother: Dorcas MAXWELL Source: none given

Ella ROBARDS Birth: 25 Jul 1815, Lexington, Fayette, Kentucky Father: William R. ROBARDS; Mother: Dorcas MAXWELL Source: none given

2a.  Census Index:  U.S. Selected Counties, 1810 (Broderbund CD-313).  Please see my WARNING regarding use of this database (the numerical data have the genders switched).  These data have more of a problem than just a gender switch; the data for males is totally incorrect (see next record). 

1810

Robards, William, Jr.

KY

Jessamine Co.

p. 57

10100-0100103

2b.  1810 Census Index and Digital Images (online at Ancestry.com; Image #23 of 28; extracted by Diana Gale Matthiesen):

1810

KY

Jessamine Co.

p. 57/600

William Robards Jr

00100-10100-03

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	16-25	1784-1794	= William (b. 1782/3)
1 female	9 or under	1800-1810	= Daughter A
1 female	16-25	1784-1794	= Dorcas (b. 1783/4)
3 slaves	all ages	in or bef. 1810	= names unknown

3.  1820

4.  1830

5.  1840

6.  1850 Census Images (online at Ancestry.com; Image #20-21 of 137):  Cole Co., MO, pp. 10B-11A, 141/143, 13/14 Aug 1850 (extracted by Diana Gale Matthiesen):

William A Robards	33	m	Lawyer (Atty Genl)	1200	Ky
Edmonia R    "	26	f			Va
[next page]
[William & Edmonia's three children]
William Robards	67	m	Farmer		Va
Dorcas    "	53	f			Ky
Ann E Robards	20	f			Mo
Dorcus   "	3	f			Mo
Anna     "	6/12	f			Mo

Edmonia is our subjects' daughter-in-law, wife of their son, William (see William & Edmonia's page for the full record).  Ann E. is also their daughter-in-law, Ann Eliza (PHILLIPS) ROBARDS, wife of their son, John M. ROBARDS — where is he?

7.  James Harvey Robards.  1910.  History and Genealogy of the RoBards Family.  Self-published, Whiteland, Indiana (photocopy courtesy of Gerald Lawrence; transcription by Diana Gale Matthiesen).

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

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

10.  Messages in the Robards Family Genealogy Forum (online at GenForum.com).