In My Case I had told the officials in ktm airport as thru' checking that I will be collecting my luggage in my port of entry in US.After that yeah In Delhi for some time the airways people take your passport with them but will return after some time  .And In my case they just make sure if the luggage was mine but I needn't check in again just recognize the luggage of mine .No,Problem at all
SO,If you say ur parents  to do through checking in KTM then there would be  no problem , atleast in my case there was no problem at all.