Perhaps others can help you with specific verses.
The whole matter depends on the spiritual maturity of the couple that you are dealing with.
If they were steeped in scripture, you might not have such a difficulty with what you’re asking.
There are so many quotation that hits at different levels of the discussion.
In the book of Job, Job loses his family, wealth, houses, etc. He says “The Lord giveth and the Lord taketh away. Blessed be the name of the Lord.”
And the general theology of the Bible, that our ways are not God’s ways, and his ways are not our ways. Our thoughts are not his thoughts.
At another level of the whole matter, people are especially protective of children. But, really, they’re no different in that each faces death eventually (until the Lord comes). We seldom know ahead when our time is coming. You can’t spring that on a grieving set of parents. It’s better for people to be aware of this and what life is about by the time they are parents.
8 to 10 years ago, there was a horrible accident on the freeways north of Chicago. A truck being driven by an illegal immigrant lost a bumper or other big piece of metal. It struck the van that this couple was driving and their six kids were burned to death in the fireball.
They were Christians and sadly accounted that they were grateful for all the time that they had with their children but they realized that they were only stewards of the children, that the children belonged to God.
We should be aware of God’s presence in our lives every minute. Lots of things, most less shocking than the death of a child, happen all the time. If we say “why, God?” that’s can be good if we listen to God’s word for the answer.
When I had a kidney stone, I was embarrassed for a moment when I said “why, God?” and then I said to myself, “why not me?” Why not accept God’s will? Isn’t that what we pray in the Lord’s prayer, presumably every day? “…thy will be done…”
A couple needs to be generally evangelized and believe in God’s word to begin with. Particular verses may help, but those are only part of the seamless whole of scripture which beckons us to God.
Deuteronomy 6:4 Sh’ma Israel Adonai eloheinu Adonai echad!
This is the most popular and frequently uttered prayers of the Jews, our spiritual ancestors.
Hear O Israel the Lord is our God, the Lord Alone! You shall love the Lord with your whole heart, and with your whole soul, and with all your might.
Jesus modified this to include “with all your mind”.
The phrase “with all your might (or strength)” the Jews interpret as meaning that you will give up all you have for the Lord, money, family, possessions. You hold back nothing from the Lord. “with all you soul” means you are willing to die for the Lord.
The sh’ma is what a Jew is supposed to say when he is dying.