My parents are planning a visit here in US in May,2018. I want to purchase roundtrip ticktets for them from KTM TO DC. My kids want to accompany them on the same return flight from DC TO KTM in June 2018, but I will have to do roundtrip ticketing for them,as they are returning in Sep2018. Can anyone share their experience on how to do this kind of same flight# ticketing ?

You will have to create a document authorizing your parents to travel with your children. Travelling from DC to ktm won't be a problem as they are with adults. Coming back is another story in itself. I don't have any idea if minors are allowed to travel international by themselves.