Get help. The real answer depends on how old your children are and how close you are to them. Often it helps to talk to a pediatric social worker or psychiatrist (or your pastor/clergy member etc) in order to correctly frame it, but there is no easy, right, or perfect way to break bad news to children. Get help, do a dry run, ask your pulmonologist to join you.
Never an easy way. The best way is to be straight with them. You would be amazed how much intuition children have and how much they know, even if you have not told them. Though it is a hard conversation, your physician may have a social worker and/or psychologist who works in the clinic for your advance lung disease and may be very helpful in facilitating this conversation.
Difficult... I think what you tell them has a lot to do wih their ages. You would need to give them age appropriate information and answer any questions/concerns they relay to you in an age appropriate way. Your physician is a good resource (as well as your kids' pediatrician if they are very young). You could also get some family counseling to help you. Best of luck to you and your family.