The reductio requires that you establish that a particular conception of goodness is essential to Christianity. I think it's a mistake to just say "no problem, it's just a reductio." Even if you argue that a theist must accept an objective standard of right and wrong, you then have to show that the standard that God supposedly violates by allowing the type of evil you are highlighting is a standard that Christians, in virtue of being Christians, are committed to. That's a bit of a demanding chore, in my book.
If you're a subjectivist, you can't say "This is the true standard of right and wrong, God violates that in virtue of allowing the evil he does allow, therefore, an omnipotent being, if he exists, can't be good." What you have to say is that Christians are committed to the standard that God is violating. Showing that commitment on the part of Christians is bound to be difficult.
A two or three years back on DI I got into some dialogue with Calvinists, in which I argued that a God who predestined some to heaven and some to hell would not be a good being in any recognizable sense. I still think that's right, but they argued that what it is for God to be good is that God's actions promote his own glory, and by glory they mean that God acted in such a way as to be able to exercise as many of his attributes as possible. God's goodness, as they understood it, required him to required him to exercise both his merciful forgiveness of sinners, which he does by giving them saving grace and welcoming them into heaven, but also by leaving people in sin and exercising his attribute of hostility to and punishment toward sin, which he exercises by punishing people eternally in hell. The Calvinists I was discussing with denied that they were theological voluntarists. God is seeking glory in this sense is, on their view, satisfying an objectively true standard of ethical conduct. Nor would I make the case that Calvinists aren't Christians.
I still think that this leaves us with too big of a disconnect between goodness as we understand in human relationships and goodness as practiced by God. But making that case as someone who believes in an objective moral standard is difficult enough. Making such a case if you are an ethical subjectivist strikes me as being just plain impossible.