Would a good God prevent WWII?

As I discuss from time to time, atheists raise contradictory objections to Christianity. Here's another example:

i) On the one hand, atheists say that if there were an omniscient, omnipotent, and benevolent God, no child would ever die of cancer (or whatever).

ii) On the other hand, atheists say that if there were an omniscient, omnipotent, and benevolent God, then horrendous evils like WWII would never occur.

But let's think about that for a moment. What would be the simplest way to prevent WWII? If Hitler died as a child, or his would-be mother or his would-be grandmother, then there'd be no Hitler, no Third Reich, no WWII. The death of Hitler as an infant, or any of his linear ancestors as infants, would preempt WWII at one stroke. 

So, if you think God ought to prevent WWII, then God ought to let children die of cancer (or whatever). Namely, children whose existence will be a necessary condition for WWII to eventuate. That's the most economical preventive measure. One death to save millions. 

(I'm not endorsing consequentialism. I'm just responding to the atheist on his own grounds.)

And while that's just one example, the same can be said for genocidal dictators generally. Therefore, a consistent atheist can't object in principle to the death of children.  They can't object to God permitting atrocities whose occurrence depends on the perpetrator or one of his lineal forebears surviving to adulthood while they simultaneously object to God permitting the death of children in principle.  

Conversely, we could turn that around. Suppose Hitler (or one of his lineal ancestors) had a life-threatening illness as a child. And suppose God let the child die. That would forestall WWII. 

Would the world be better off in that event? That depends. From what I've read, the reason we developed the bomb was fear that Germany would get the bomb first. Indeed, they had a head start. As it turns out, their experiments were a dead-end, but we didn't know that at the time. Because we were afraid they might beat us to the punch, FDR authorized the Manhattan Project. 

Suppose, though, Hitler never existed because he or one of his lineal forebears died in childhood. That would sap the urgency for us to develop the bomb. Absent that catalyst, what if Russia or China got the bomb before we did. They could then use that as nuclear blackmail to impose Maoism or Stalinism worldwide. And that would be even worse that WWII. 

For atheists, if children die, that's evidence for the nonexistence of God. Yet if some children don't die (e.g. baby Mao, Pol Pot, Genghis Khan, Hitler, Stalin)–resulting in massive, horrific evils–that's evidence for the nonexistence of God. But in that case, atheists have contradictory objections. Letting children die disproves his existence while not letting children die disproves his existence!  

There's the further paradox that if God preempted some massive atrocity by permitting the perpetrator (or a lineal forebear) to die in childhood, there'd be no evidence that God preempted that eventuality. God never gets credit for a nonevent. For all we know, God has, in fact, prevented many a Mao, Stalin, Hitler, Pol Pot from rising to power because they died in childhood. No one remembers because they didn't live to do anything memorable.