There were two boys born FIELD with a mother's maiden name WHIDDETT in Q4 1850 and Q2 1851. They would therefore be 10 and 9 (or very close) by the time of the 1861 census. I believe the unnamed one became Frederick and on the census their names were inadvertently flipped so they had the wrong ages. This is supported by their birthplaces of Blean and Canterbury.
FIELD, - WHIDDETT
GRO Reference: 1850 D Quarter in BLEAN Volume 05 Page 25 Order
FIELD, WILLIAM HENRY WHIDDETT
GRO Reference: 1852 J Quarter in CANTERBURY Volume 02A Page 474
Unfortunately Frederick seems to have been born in the Union Workhouse at St Mary Bredin. His birthplace is given as Herne which falls under the Blean registration district. There is also a 7-year old Harriott Field and another sibling Osborne Field.
"England and Wales Census, 1851," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:SGLX-5L1 : 9 November 2017), Frederick Field in household of Osborne Field, St Mary Bredin, Kent, England; citing St Mary Bredin, Kent, England, p. 23, from "1851 England, Scotland and Wales census," database and images, findmypast(http://www.findmypast.com : n.d.); citing PRO HO 107, The National Archives of the UK, Kew, Surrey.
If you subscribe to FindMyPast you can view the image and go back a page to find Edward and Harriott Field, both 30.
Edit: And here's the baptism of Osborn Widdet [sic] showing his mother was Harriet.
"England Births and Christenings, 1538-1975," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:J3MX-1Q8 : 11 February 2018, Osborn Widdett, 06 Jun 1841); citing , index based upon data collected by the Genealogical Society of Utah, Salt Lake City; FHL microfilm 355,633.