It is a tough thing. My heart and tears are with you right now as I read your post. No one should have to lose their grandchildren in such a way.
As one who strongly believes in the Word-Faith message, I have no doubts in my mind that what the Bible says about healing is true. But in all honesty, our life experiences do not always seem to match what the Bible says we should have. I do not believe that the fault is God's, and it is not always ours either, but there are so many other contingencies that we are dealing with.
We are in a warfare, and in this warfare there will be casualties. The painful thing is when one of the casualties happens to be one that we love and treasure dearly.
Sometimes we Word-Faithers have attempted to be "Super-Faith Men". We have tried to take on the devil and his attacks on our own without calling for help. Some healings simply will not come by our prayers and confessions alone. Sometimes it will take many of us, joining our faith together, praying and interceding, not quitting until what we are seeking God for has come.
We Word-faithers have sometimes thought that a quick prayer, Scripture quote, rebuke of the devil, and positive confessions hereafter would get the job done. I am learning that there is more to it than that. We have sometimes forgotten that there is the aspect of INTERCESSION and also the Matt. 18:18-20 principle. Older Faith teachers such as Gordon Lindsay, Kenneth Hagin, John Osteen, and other pioneers taught these truths, but their voices were lost in our impatience to get the quick faith answer. Even Bob Buess, a friend of Hagin and Osteen rebuked the WoF for the lack of perseverence in prayer in his out-of-print book "As The Pendulum Swings."
From here, we do not want to ask "why" about your departed grandchild. Just rest assure that as you hurt, God is hurting with you. He loves you so much that he is feeling your pain just as you are hurting. However, why don't all of us on this forum begin to pray for your other grandchild and beseech God for a miracle.
Father
I cry out to you to glorify yourself in working a miracle on behalf of this child. Lord, the doctors have given us a negative report, but we know that you are the all powerful and omnipotent one who takes delight in showing that one need not settle for the doctor's negative verdict. We thank you Lord for the wisdom you have given doctors and how they strive to help. But Lord, we know that you are the ULTIMATE physician and you have promised in your Word to hear and answer prayer. You have promised to heal our bodies.
Lord we ask you to fulfill your powerful word on this child's behalf and HEAL in the name of your holy Son, Jesus the Messiah. Do it for your name's sake. Do it for your glory. Show the world that you are one who keeps His vows. I pray, standing on this in the name of Jesus. Amen