“Given the Leftie nature of the Dunhams, the ability of their grandson to avoid the draft by just going to Kenya for a couple of years must have occurred to them.”
He was born in 1961. The draft ended in 1973, and he wouldn’t have been 18 until 1979.
Look at it from the viewpoint of parents and grandparents in 1961, without a crystal ball. WW2 and Korea was just a few years in their past. They would have seen a significant probability of a war with the USSR at some point in the future. In that world, having a son with Kenyan citizenship might not seem such a bad idea.