Over 70 Years
Queen Elizabeth and Prince Philip do not make a lot of physical contact in public, but the two of them have been together for over seventy years by now. This is all the evidence we need to figure out that PDA is not a requirement for a successful relationship.
The Kids Included
Even though it is not really common to see royal couples get all touchy-feely, perhaps people approve of such behavior between children and parents more. Although it varies from one parent to another, history shows that this does not happen all that often either.