I agree there may be a mistake in the writing up of the bapt records.
1841 Charles is with John/Sarah and in 1851 are down as his Grandparents as you know.
Closest Birth Reg I can see;
COLVER, CHARLES mmn PHILLIPS
GRO Reference: 1841 M Quarter in HERTFORD UNION Volume 06 Page 519
Sarah PHILLIPPS
John CALVERT married 29 Jul 1815 All Saints, Little Munden
(FreeREG)
John/Sarah bapt;
Use soundex as surname is Calvert or Culver
Mary Ann 25 Feb 1816 LM
Eliza 07 Sep 1817 LM
Martha 06 Dec 1818 LM
Elizabeth 03 Dec 1820
James 15 Sep 1822 W
Ann 06 Jun 1824 LM 6th child, Mother late Philips spinster
George 18 Dec 1825 LM, Mother Philips
William 28 Oct 1827 W Father only named
Emma 05 Jul 1829 W
Joseph 10 Apr 1831 W
Sarah 26 May 1833 W
Sonja 07 Jan 1838 W *** likely Louisa Colvert reg Dec 1837
LM Little Munden, W Walkern
WILLIAM HENRY Culver bapt 04 Apr 1841 Walkern, Mother ELIZA, spinster
All above on FreeREG
So in 1851 William is down as John/Sarahs son but is their grandson, so is Charles noted incorrectly and should be their Son****.
I definitely think so.
Birth Cert reg seems to fit that scenario.
Trish

EDIT;
I did notice that 1851 has g son for Charles but enumerator has put Grand Daug/Son in Law for the next family (using capital letters).