I knew about it and it is listed in opening credits but here is the story in wiki.....
Heston’s newborn son, Fraser (born February 12, 1955), was cast by DeMille (on the suggestion of Henry Wilcoxon, who said to him “The timing’s just right. If it’s a boy, who better to play the Baby Moses?”) as soon as Heston announced to DeMille that his wife Lydia was pregnant.[19] Fraser Heston was three months old during filming.
Might as well...