How do we use "have" or "had" when referring to the number of offspring that a couple produced?
The basic case is when a couple gave birth to, say, three children and everyone is still alive. "They have three children."
What if the parents both died? "They had three children." They are dead so they can no longer have anything. What if only one parent died? I expect that again that we'd use "had" because while they did have the children together, they have nothing together anymore because one of them is dead.
What if all three children died? "They had three children." They no longer have any children. Does this change if not all of their children died? Again, "had" seems more appropriate.
Maybe it depend on exactly what we mean by the terms. Maybe not. Do we refer to the moment when they gave birth to the children or to their act of continuously raising or having raised them. If we speak of birthing, then maybe it should always be "had" because it's always a past event.