Children of Willem Gert Johannes Slabbert (1894-1974) and Helena Cornelia Fredrika (Pretorius) Slabbert (1902-1960)

Hi, can anyone know or know how to find out who all the children names of Willem Gert Johannes Slabbert (1894-1974) and Helena Cornelia Fredrika (Pretorius) Slabbert? my dad says they had 13-14 children, I only found baptism records for their 2 youngest, Hermanus Edward (born 1945) and Willem Gert Johannes Sabbert (born 1944). Willem Gert Johannes is my grandfather but he passed away in 2008 so I can't ask him what all his siblings names were, I know there was a Sina, a Siel (these are probably nicknames cause there was a Francina too), Hermina I think was one of the names, my dad says he can't remember all their names and I think all of them would be dead by now, they would've been born sometime 1921-1946. and I wish I had photos for them all lol