His daughter Kezia was registered as Kezia Davis Fluck which leaves no doubt about her parents but I can't find similar Fluck births for George and Sarah. Could Walter have been a widower when he married Eliza Fluck?
"England and Wales Birth Registration Index, 1837-2008," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:2N34-WGX : 1 October 2014), Kezia Davis Fluck, 1848; from "England & Wales Births, 1837-2006," database, findmypast (http://www.findmypast.com : 2012); citing Birth Registration, Westbury On Severn, Gloucestershire, England, citing General Register Office, Southport, England