The implication of Margaret not going by surname SIMMERMAN in the censuses is that she and Christopher were never married.  That the children were surnamed SIMMERMAN indicates she may have sued him for bastardy and gained for them the legal right to bear his name or that he simply acknowledged them as his children.  County court records, if they are extant, may shed light on their relationship, especially if Christopher was deceased because minor children were considered "orphans," even if their mother was still alive.  Because the mother had no legal rights, children whose father was deceased had to have a male guardian appointed to them by the court, someone with the legal power to watch over the rights of the children.  I see from the library index at the LDS web site that the orphan's court records of Wythe County have been compiled and published by Mary B. Kegley (c1995) of Wytheville; this source (on microfilm at the LDS) would definitely be worth consulting.

I find it very hard to accept that Robert is Margaret's biological son because it would mean she birthed a child at the age of at least 45.  It's far more likely that he's Delilah's child.  It was not uncommon for a mother to claim a daughter's illegitimate child as her own to protect her daughter's repution.  Or was Robert adopted?

1.  Marriage Record:

2.  Messages posted to the Simmerman Family Genealogy Forum (online at GenForum.com), including this excerpt:

Christopher A. Simmerman 29 yrs b Wythe Co., VA s/o Stuffle Simmerman and Peggy Stroup, md June 17, 1867 in Wythe Co., VA Book 2A p#51, Eliza R. Fisher 27 yrs b Wythe Co., VA d/o Rev. Absolam Fisher and Juda Rader...

Secondary sources show the marriage in 1869, but unless there's a typo in the above message, the year is 1867.

3.  1840 Census Index/Images (online at Ancestry.com):  not found.

4a.  1850 Census Every-Name-Index/Images (online at Ancestry.com, Image #98-99 of 235):  Sixty-Eighth District, Wythe Co., VA, Roll M432_982, pp. 272B-273A, 683/643, enumerated 7 Aug 1850, official enumeration date 1 Jun 1850 (extracted by Diana Gale Matthiesen):¤•

George D Findley	22	M		Labourer		Virginia
Margaret Stroupe	40	F				"
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Delila Simmerman	16	F				Virginia
Christopher  "	13	M				"

Margaret appears to have been enumerated twice (see next two Sources).

4b.  1850 Census Every-Name-Index/Images (online at Ancestry.com, Image #192 of 235):  Sixty-Eighth Dist., Wythe Co., VA, Roll M432_982, p. 319B, 1322/1322, enumerated 20 Sep 1850, official enumeration date 1 Jun 1850 (extracted by Diana Gale Matthiesen):¤-

Margaret Stroupe	48	F				Virginia
Alexander   "	10	M				"

Margaret is listed here between her mother, Mary (WENRICH) STROUPE (see next Source), and her brother, George STROUPE.  Yet she is just as surely the one above listed with the SIMMERMAN children.  Did she move between August 7th and Septemer 20th?  The enumerator is supposed to give the residence as of June 1st, but they don't always comply.

4c.  1850 Census Every-Name-Index/Images (online at Ancestry.com, Image #192 of 235):  Sixty-Eighth Dist., Wythe Co., VA, Roll M432_982, p. 319B, 1323/1323, enumerated 20 Sep 1850, official enumeration date 1 Jun 1850 (extracted by Diana Gale Matthiesen):¤-

Mary Stroupe	95	F				Pennsylvania
Mary    "	17	F				Va

Could the younger Mary be our Margaret's daughter (a.k.a., Delilah), helping out by living with her maternal grandmother?  If not, then Delilah was missed on the second go-round.

5.  1860 Census Every-Name-Index/Images (online at Ancestry.com, Image #216 of 223):  Wytheville P.O., Sixty-Eight Dist., Wythe Co., VA, Roll M653_1385, p. 97, PN 251, 1862/1673, enumerated 6 Aug 1860, official enumeration date 1 Jun 1860 (extracted by Diana Gale Matthiesen):¤•

Alexander Simmerman	20	M		Laborer		40	Virginia
Margaret Stroup	54	F					do
Delilah	25	F					do
Robert	5	M					do

6.  1870 Census Every-Name-Index/Images (online at Ancestry.com):  can't find Margaret, Delilah, or Robert, and I really looked.  There is a Margret SIMMERMON (æ 66, b. VA) in Fort Chiswell, Wythe Co., VA (p. 454B), but she's the widow of John A. SIMMERMAN.

7a.  1880 Census Every-Name-Index/Images (online at Ancestry.com, Image #6-7 of 42):  Wythe Co., VA, Roll T9_1395, pp. 480B-481C, PN 6-7, SD 5, ED 111, enumerated 4/5 Jun 1880, official enumeration date 1 Jun 1880 (extracted by Diana Gale Matthiesen):¤•

1880:  for an explanation of the column headings, please see What the Numbers in the Federal Census Mean (missing columns contained no data).
1	2	3	4	5	6	8	9	13	22	23	24	25	26
55	55	Stroup Peggy	W	F	80		/	levok?			VA	VA	VA
		"   Delila	W	F	45	Dau	/	wash woman			VA	VA	VA
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55	55	Stroup Robt M	W	M	26	Son	/	Farm hand	/	/	VA	VA	VA

Note that Peggy's marital status (Column 9) is single, not widowed or divorced.

7b.  1880 Census Every-Name-Index/Images (online at Ancestry.com):  C.A. SIMMERMAN (æ 43, b. VA) and his wife, Elizabeth R. (æ 41. b. VA), are living in Black Lick Dist., Wythe Co., VA (p. 365B), with their five children.

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