An interesting addition here would be the first father/Son winners in a Grand Prix, or major international race. On the European front my candidates would be the duo of Antonio and Alberto Ascari, and the year 1948 when Alberto won the San Remo GP. There may well be others.
On the American side, I don't see any Indy combinations as early as 1948, but there are many other series I am unfamiliar with. Stan, do you know any father-son winners in a major race, before 1948?