Hello Eds,
I can't find them on the 1841 either, however ... I realise Sarah's surname is transcribed as SPARKS on Free BDM, but the children (except Myra who I can't find) have mother's maiden name PARKES (or PARKS) on the GRO search.
Henry CASTLE mmn PARKS 1842 J Qtr RADFORD (15/621)
George CASTLE mmn PARKES 1844 D Qtr RADFORD (15/604)
Mary CASTLE mmn PARKES 1850 D Qtr WALSALL (17/235)
(Can't find Ann's registration?)
Charlotte CASTLE mmn PARKES 1854 S Qtr WALSALL (06B/472)
Esther CASTLE mmn PARKES 1857 J Qtr WALSALL (06B/562)
Modified to add: good find Kay
