1.  Marriage Record:

2.  1850 Census Every-Name-Index/Images (online at Ancestry.com, Image #1 of 83):  Town of Boonville, Oneida Co., NY, Roll M432_565, p. 150A, 3/3, enumerated 22 Aug 1850, official enumeration date 1 Jun 1850 (extracted by Diana Gale Matthiesen):¤•

Richard H Platt	33	M		Cooper		NY
Phidelia    "	23	F				"
Willard F   "	2	M				"

Their son is definitely, "Willard F."

3.  1860 Census Every-Name-Index/Images (online at Ancestry.com, Image #26 of 39):  North Western P.O., Steuben, Oneida Co., NY, Roll M653_825, p. 402, PN 172, 1323/1319, enumerated 10 Jul 1860, official enumeration date 1 Jun 1860 (extracted by Diana Gale Matthiesen):¤•

Richard H Platt	43	M		Farmer			NY
Fidelia     "	33	F					"
William F   "	12	M					"
Margaret A  "	9	F					"
Esther	5	F					"
George H	1	M					"

The eldest child is definitely "William," and George's middle initial is definitely "H," both of which appear to be wrong (see obit below).  Listed next to Fidelia's brother, George H. STROUP.

4.  1870 Census Every-Name-Index/Images (online at Ancestry.com; Image #9 of 32):  North Western P.O., Steuben, Oneida Co., NY, Roll M593_1059, p. 717A, PN 9, 72/72, enumerated 21 Jul 1870, official enumeration date 1 Jun 1870 (extracted by Diana Gale Matthiesen):¤•

Platt Richard	60	M	W	Farm Laborer			N York
"   Delia	40	F	W	Keeps House			N York
"   Esther	16	F	W				N York
"   George	11	M	W				N York
"   Almira	8	F	W				N York
"   John	6	M	W				N York
"   Phidelia	4	F	W				N York
"   Mary	2	F	W				N York
"   Polly	84	F	W				N York
Stroup Margaret	65	F	W			200	N York

Polly is Richard's mother and Margaret is Delia's mother.  Listed five households from Delia's sister, Susan (STROUP) MOWERS.

5.  1880 Census Every-Name-Index/Images (online at Ancestry.com, Image #5 of 27):  Steuben, Oneida Co., NY, Roll T9_903, p. 189A, PN 5, SD 8, ED 121, enumerated 4 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	10	13	24	25	26
41	44	Platt Richard	W	M	62			/	Farm Laborer	NY	NY	NY
		_____ Adelia	W	F	52	Wife		/	Keeping house	NY	NY	NY
		_____ Mary	W	F	11	Dau	/		At school	NY	NY	NY

Listed next to Delia's sister, Susan (STROUP) MOWERS.  Daughter Phidelia PLATT (æ 14, b. NY) is living as a servant with George & Susan ROGERS in Western, Oneida Co., NY (p. 383A).

6.  1890 Census:  the 1890 Census Population Schedules were destroyed.

7.  1900 Census Every-Name-Index/Images (online at Ancestry.com, Image #16 of 19):  Steuben Town, Oneida Co., NY, Roll T623_1132 (Book 2), p. 8B, SN 8, SD 13, Ed 47, enumerated 19 Jun 1900, official enumeration date 1 Jun 1900 (extracted by Diana Gale Matthiesen):¤•

1900:  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	7	8	9	10	11	12	13	14	15	22	23	24	25	27
182	190	Platt Richard	Head	W	M	Sep 1816	83	M	53			NY	NY	NY	Y	Y	Y	R	H
		_____ Fidelia	Wife	W	F	Oct 1826	73	M	53	10	8	NY	NY	NY	Y	Y	Y

Listed next to Fidelia's nephew, John H. STROUP, and three households from Fidelia's niece, Mary (STROUP) YUTZLER.

8.  1910 Census Index/Images (online at Ancestry.com):  not found.

9.  Unsorted Obituaries (online at the Oneida County NYGenWeb site; this one courtesy of Adelaide Webster):

Rome Sentinel November 23, 1905 Mrs. I. A. Webster, 124 Park Street, has received the sad word of the death of her sister, Mrs. Almira Cronk of Boonville, widow of Milton H. Cronk, aged 44 yrs.  Member of the M. C. Church of Boonville, and christian worker and beloved by all who knew her.  Most of all, where she leaves three children to mourn, the kind loss and loving mother, also her aged father, Richard H. Platt of Boonville, three brothers, Willard F. of Wright Settlement, George E. of Forrestport, John E. Platt of Ava, four sisters, Mrs. David Money, Mrs. I. A. Webster of Rome, Mrs. Walter C. Moyer of Richfield Springs and Mrs. William Penn Griffith of Boonville.

Apparently, Fidelia is deceased.

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

11.  Messages posted to the PLATT Surname Board (online at Ancestry.com).

12.  Messages posted to the Platt Family Genealogy Forum (online at GenForum.com).

13.  Email from Carl and Lorry Berndt, who (among other things) alerted me to the obituary in the Rome Sentinel.

14.  Emails from Dick Platt.