How to tell my children

My husband has terminal pancreatic cancer, he is very close to the end of his journey. we have 2 young children age 3 and 7. Both my children have communication difficulties due to deafness, we have tried to explain what is happening particularly to my 7 year old we have used books movies even our macmillan nurse has tried to explain he doesn't get it, how do I tell him where his daddy has gone when the time comes.