Parents of Muggle-borns
We know that certain magical places have protection around them to keep the Muggles from finding out about the wizarding world, however, what happens when the parents of a Muggle-born (thus already knowing of and being exposed to magic) enter a magical dwelling? What would happen when they entered the Leaky Cauldron, or Hogwarts? Would they see themselves entering an old ruin, and then what happens once they're inside?
And Hermione's parents enter Diagon Alley, would they definitely need the presence of Hermione to do so?