If you are asking this through tears, please know: the Church does not have a tidy answer that fits on a card, and would not insult you with one if it did.
What Christianity refuses to say
That the child deserved it. That it was God's will in a punitive sense. That suffering is illusion. That a "greater good" cancels the cost. Job's friends tried those answers, and God rebuked them.
What Christianity does say
Sickness, accident, and death are intruders in God's world — consequences of a creation in bondage to corruption (Rom. 8:20–22), not God's design. When Jesus met grief, he did not lecture; he wept (John 11:35). And then he raised the dead.
The Orthodox lens
The Holy Innocents — the children of Bethlehem killed by Herod — are commemorated in the Church's calendar as the first martyrs. The icon shows mothers weeping. The Church does not hide their suffering; it places it next to Christ's manger. God's first companions on earth were grieving mothers.
St. Gregory of Nyssa wrote a whole treatise (On Infants' Early Deaths) wrestling with this. His answer is not philosophical comfort but eschatological: the God who took a child into his arms (Mark 9:36) will not lose one of them.
A pastoral word
Tim Keller, after losing friends to cancer, said: Christianity does not give you a reason for every suffering, but it gives you a Person in every suffering — one who knows what it is to be a child crying out and unheard. The Resurrection is God's promise that he will undo this, down to the smallest grave.