I'm so sorry to hear this, Cathmary. It is a parents' greatest fear, and a grandparents' too.
I think the previous posters are probably right when they say that your daughter and her husband are probably in shock at the moment, and are struggling to find a way to cope with such devastating news. Their needs and wants will probably change over time, and it is very probable that a time will come when they will need both practical and emotional help from you whilst nursing the little girl. My only experience of caring for a cancer patient involved an adult (my dad) but I know that, as the illness progressed, my mum and I were increasingly grateful for any help.
However, the realisation that things will probably change doesn't help you now with your feeling that you want to be there for them and to spent precious time with your granddaughter. It's perfectly understandable and I think all you can do is to do what you can in practical terms, such as helping with the other children if possible, and also to allow yourself to grieve. Grief doesn't start when someone has passed away, it begins when you know that are going to lose them, and you need to let yourself acknowledge those feelings. I hope you have people around you who can support you and comfort you.
Once again, I am so sad for your situation.