I am looking for Cor and Cockij Barth - They were cousins to my grandparents Corstiaan and Alida de Wagenaar.  I am unable to determine their full names or name at birth of Cockij. I think they had 1 son 

Which period? Living area?

kind regards, Ben

I don't know if these are the right ones. Zoekresultaten ('s-Gravendeel) » Open Archieven

Cornelis Barth, born Dec. 19,1893 in 's-Gravendeel, son of Dirk Barth and Janna Tak. Cornelis had a sister Cornelia Barth, who married on Aug. 10, 1901 in 's-Gravendeel (at the age of 21) with Cornelis Timmers.

Cor Barth was son of Lijntje de Wagenaar (Barth) Born 5 Apr 1900 in Dordrecht Wife of Nicolaas Hendrikus Joseph Barth — married 15 Jan 1920 in Dordrecht, Zuid-Holland, Nederland.

Cockij (may have been Cornelia) Barth could have been daughter of one of Lijntje's brothers (de Wagenaar).  They were cousins that married and had one son born about 1968.

I am unable to find any records.  

@Estelle, [children unknown] here of Lijntje de Wagenaar & Nicolaas Hendrik Joseph Barth, are here to be found (registration card)

update after reading the above: the card gives no son Cor

the family so far on familysearch: 

Nicolaas Hendrikus Joseph Barth, zoon van Hendrikus Philippus Barth en Antonia Boerboom, huwt in Dordrecht op 15.1.1920 met Lijntje de Wagenaar, dochter van Willem Johannes de Wagenaar en Cornelia Versluis.

Cornelis Johannes Barth - Online Familieberichten 

Mevr. C.M. Barth-de Wagenaar werd op 18-2-1986 herkozen als secretaresse van de plattelandsvrouwen in St. Philipsland. Eendrachtbode (1945-heden)/Mededeelingenblad voor het eiland Tholen (1944/45) | 27 februari 1986 | pagina 15 - Krantenbank Zeeland