Christians: how would you explain to the young siblings of a child born with a birth defect what has happened?

Say, for example, a baby has just been born blind; its eyes didn’t form properly during gestation. There’s nothing the doctors can do and though you pray wholeheartedly to your invisible god, as usual, nothing happens. The other children, the siblings of the baby, ask you, “What’s wrong with the baby’s eyes?” Do you opt for the rational explanation about random mutations and occasional unpredictable congenital birth defects or do you say, “God did it and God has a reason for doing what he does blah, blah, blah...”?