Imagine in 1900 if people had some technology that would make the earth vanish into nothingness in 2050, after everyone currently alive had died. Would it have been morally neutral for them to deploy that technology, since the only people it would affect are nonexistent hypothetical people?

On your specific points:

> you have to also count and subtract the deaths they will cause

Agreed.

> A few of those children might turn out to be be murderers. Others will cause accidents, maybe big ones. Others still will have consumption habits that snuff out future lives.

That's right: each person that comes into existence has some chance of making the world worse for others, and some chance of making it better. I think that overall new people are a positive contribution, especially if you consider their own enjoyment of life, but if you think it's the other way then it's completely to reasonable count "more people will be born" as a negative feature of a policy.

> Imagine if Adolf Hitler's mother had died in some accident before he had been conceived and born; how would you tally the accident?

When looking at moral questions you're generally concerned with decisions: do I try jump in to help with this accident in front of me? Do I give money to this person that seems to need it more than I do? Do I advocate for this policy? It's not clear to me what sort of moral question your hypothetical corresponds to, since at the time of the accident (or before that if we're considering the value of some sort of policy that might have affected the chance of her dying) no one knows that it's her child in particular that would end up causing such enormous harm.