The only people who can really speak to this issue are people who have R”L lost a baby. I doubt there are many people who would welcome such a “nisayon” that makes them “stronger.” Many people do NOT become stronger, Marriages fall apart, people have nervous breakdowns, and the rest of the family dynamic is forever altered.
The only right thing to do is BE THERE for the grieving parents, cry with them, help them with mundane chores, shop for them, cook for them, and watch their other kids, if any. Above all, keep your platitudes to yourself. Perhaps some extraordinary people want to hear them, but most parents are not in a place where that is a comfort in any way. The worst thing you can say is stuff like Hashem only gives tzoros to people who are strong enough to handle them. You think THAT gives someone chizuk???? My husband’s sister lost her daughter at age 12, very suddenly. DO you know that there were some incredible blockheads who told her, “You’re young enough to have another baby.” Seriously??? That’s what you tell a woman who has just lost her beloved daughter? It happened that she was NOT able to have more children (she had another younger daughter, but couldn’t conceive again). But so what – does a new baby discount the older one? People are beyond insenstive sometimes. It never ceases to amaze me.