You can sit down your parents at the CheckIn in NCL and take their passports to getting the boarding passes at the airport. By this you could keep their boarding pass for LHR-JFK until going airside in LHR again. Just tell them that you have to take the passports at first wing as of ‘inviting them to the lounge as a Gold member’.
I’m doing it like that if I want to surprise somebody and always say I’m going to manage the passport things for them