Scarlet fever was caused by strep, easy to treat with antibiotics.
So it the sustained human project of helping each other, that’s what brought medical science, a middle-class lifestyle, good jobs for most of us, not quite enough on that count, etc.
But imagine, just about 100 years ago, losing a child to a disease which is now thoroughly treatable. Weirdly, strep bacteria can of adapts in how it attacks the human body.
God's plan does not always align with our plans. It is simply not God's will for every child to reach adulthood. It's not God's will for every young adult to reach old age. But it is God's will for everyone to be saved so they can live the perfect life when this earthly journey is over.
If we think about it, everyone will eventually face the same outcome - death. We all accept this and don't question or blame anyone for it nor expect God to prolong our lives forever. The Christian is in a unique position to say we're not guaranteed anything so if a child dies, it's not different than an old person dying in God's eyes. The Christian can also fall back on the next life not having any diseases, suffering, etc. It's not a fullproof defense against the 'problem of evil' arguments, but it's enough to leave an opening or potential path to a good response, imo.