Off course you have the right to wail and whine - this IS your family, who (including yourself) have already and are soon to have their lives torn apart even more. It may not seem so at the moment but you are luckier than some who are also faced with 'the worst' happening but because of circumstances cannot offer any practical help or support. You do have that opportunity - to keep life as normal as possible for the children, to give them extra cuddles, a bit of leeway, (they will know something is wrong), reassure them and be a shoulder to cry on/lean on for your son.
With the summer holidays looming, you could investigate playschemes, sports courses etc. which would keep the children happily occupied and give you a break. Also think you need to get your DH on board so that you have back up for yourself. Maybe show him this thread? Tap into any available support e.g. Mcmillan Nurses.
Finally, you will cope, you will be amazed at how you can carry on, you will wonder where you found the strength. Just as love grows and expands to encompass a new child or grandchild, it will swell to allow you to support your family through this awful time.