I found the psalms and prayers of the small paraklesis to the Theotokos to be a comfort. I try to pray this at home in times of need, but it seemed especially fitting for this situation:
and to Him do I proclaim all my sorrows,
for many woes fill my soul to repletion,
and lo, my life unto hades has not drawn nigh.
Like Jonah do I pray to Thee.
Raise me up from corruption, O Lord, my God.
Most holy Theotokos, save us!
My nature held by corruption, and by death
hath He saved from out of death and corruption,
for unto death He, Himself, hath submitted.
Wherefore, O Virgin, do thou intercede with Him
Who is in truth thy Lord and Son,
to redeem me from enemies’ wickedness."
- from Ode 6 of the Canon of the Paraklesis
The suffering of children easily reminds us the prophecy of St. Simeon to Christ's mother: "and a sword will pierce through your own soul also." God sees our grief, especially the grief of mothers who are so intimately connected to our children.
This is the mystery of our union with Christ and with one another in the body. We are not isolated individuals - we are connected. When we love as Christ loved, and we suffer for the love of one another, we share in the sufferings of Christ as his mother did. There is a story of a grieving couple who lost a child. In the face of their grief, their priest didn't offer platitudes, but told them to take flowers to the Mother of God each week, "Stand in front of her icon, or kneel there, or roll around on the floor - I don't care. Scream, yell, cry, curse, I don't care, but go and be with her. She is the mother of our Lord, and she knows." (Forest, The Ladder of the Beatitudes, pg. 40)
And so, I don't have answers. I am grieving and praying with this family, and beseeching the prayers of the Mother of our Lord for this mother, also. Lord have mercy!
Answer me speedily, O Lord;
My spirit fails!
Do not hide Your face from me,
Lest I be like those who go down into the pit.
Cause me to hear Your lovingkindness in the morning,
For in You do I trust;
Cause me to know the way in which I should walk,
For I lift up my soul to You.
- from Psalm 142 (143)