This made me cry today.

Here is a passage from John Flavel's book, 'Facing Grief' that was helpful for me:

"Before this relation whose loss you lament came to know life, the time of your enjoyment and separation was unalterably fixed and limited in heaven by the God of the spirits of all flesh; and although it was a secret to you while your relation was with you, yet now it is a plain and evident thing that this was the time of separation before appointed; and that the life of your relation could be no means be protracted or abbreviated, but must keep you company just so far, and then part with you.

This position has full and clear Scripture authority for its foundation. How full is that text in Job 14:5, "Seeing his days are determined, the number of his months are with You, You have appointed him his bounds that he cannot pass." The time of our life, as well as the place of our habitation, was fixed from all eternity past. It will greatly conduce to your settlement and peace to be well established in this truth - that the appointed time was fully come when you and your dear relation parted - for it will prevent and save a great deal of trouble, which comes from our after-reflections. Oh, if this had been done, or that omitted; had it not been for such oversights, etc. my relation would be alive this day! No, no, the LORD's time was fully come, and all things concurred, and fell in together to bring about the pleasure of His will. Let that satisfy you: had the ablest physicians in the world been there, or had they that were there prescribed another course, as it is now so it would have been when they had done all..... The LORD's time was come; your relation lived with you every moment that God intended them for you before you had them with you.
And let none say the death of children is a premature death. God has ways to ripen them for heaven whom He intends to gather there eventually, which we know not. In respect of fitness they die in a full age, though they be cut off in the bud of their time. He Who appointed the seasons of the year appointed the seasons of our comfort in our relations; and as those seasons cannot be altered, no more can these. All the course of providence is guided by an unalterable decree; what falls out casually to our apprehension yet falls out necessarily in respect to God'd appointment. Oh, therefore be quieted in it. This must needs be as it is."
 
Thank you, dear friend for sharing with me what has comforted you in the past.  This is also a good reminder as we contemplate God's soveriegnty in our children's lives.