Last name of father and mother, respectively.

  let motherLastName = "Carter Hughes"
  let fatherLastName = "Miller Thompson"
  let childLastName  = "Miller Carter"
  let childFullName  = "Jean Paul Miller Carter"

You might ask, —“Why does the father’s last name go first and the mother’s second?”— That’s an old tradition, and it can change whenever enough people in our society agree. As it stands, the father’s family name tends to persist down the family tree, while the mother’s family name often disappears in each generation.

Or so that is how it works in many countries around the world.

You should have given a more complete example, where the parents themselves have long names to demonstrate that something does have to get dropped when you have children.