When I was little, I briefly lived in a house with 3 generations and 10+ people in a single room.
From what I remember, parents would just pick a time to be alone, and grandma would sit on the porch and shoo any curious children away from the house.
Gramms always knew best — in my teenaged years, I briefly lived with my maternal great grandmother; she would always suggest we "go neckin'," which to her was the polite way of her proclaiming: sire her great-great grand-children.
From what I remember, parents would just pick a time to be alone, and grandma would sit on the porch and shoo any curious children away from the house.