What Is The Reason For Taking A Dpt Shot?

my daughter is having a baby in a few weeks and her dr said her and her husband should get the dpt vaccine along with all those who will be in direct contact for long periods of time with the baby. they live with us so I am assuming our family should get the dpt shot also, true? my younger children are 17 and 18, is there dpt shot they received when they were 5 still effective or do they need another one?