Telling the grandchildren

My dad is 73 and has recently been diagnosed with advanced prostate cancer.  It has spread to his lymph nodes so is incurable but he has started on hormone treatment. The consultant told us as long as the HT works the cancer can be controlled.  This can be anything from 18 months to 4 years.  On the positives though he feels well and has no symptoms at all - he was diagnosed 5 yrs ago but was very slow growing but somehow in the last year it started growing really fast and his PSA level shot up.  My mum is finding it really hard to deal with the whole situation.  I saw them last week and they both broke down.  I have 2 children 13 and 11 and really don't know whether to tell them or how to tell them.  He is desperate to see them but doesn't know how to and what happens if his emotions take over and he breaks down.  If I told them then at least they know why Nan and Grandad are upset.  My son is autistic and very black and white and we don't have any definitive answers of how or when to tell him.  Just wondering what other peoples advice is on telling the grandchildren.   Because he is well they don't actually need to know but from his point of view I think he may be avoided seeing them because it is too hard emotionally. Many thanks