I see a few possible scenarios:
1. God has a reason for letting bad things happen that I don’t understand. Just because I don’t understand it doesn’t mean there isn’t a reason. There are many things I don’t understand, or can’t understand even.
2. God let it happen because he doesn’t meet our measure of goodness.
3. There is no God, so he didn’t let it happen, it just happened by itself.
4. God will undo all suffering in the reconciliation.
5. Or something else I haven’t thought of.
Some Christians would argue that we know God is good on other grounds so that, even when we can’t explain how God could let a terrible thing happen, the other things that we know allow us to believe in God despite this serious challenge to that belief.
A possible scenario of conversation with God I imagine might be:
Person: why didn’t you save the child?
God: it’s okay he will be resurrected and have a good life in eternity.
Person: that’s not good enough, what about the suffering he had now, the life that he lost now, and people who mourned the loss of the child?
God: that’s okay, the future life will be so good no one will even care to remember the suffering of the past.
Person: that’s not good enough at all! No amount of forgetting can undo the terrible suffering here and now.
God: okay listen up. If you really want to know then this is how it is. I won’t just provide a blissful future. And I won’t just make people forget the terrible suffering of the past. What I will do instead is I will go back in time and stop the child from being killed. Is that good enough?
Person: I suppose, but why did you let it happen at all?
God: if I go back and stop it from happening then it never did happen.
Person: can you really do that? It doesn’t seem to make sense.
God: I’m God, what do you think? It’s no more difficult for me to prevent an accident last Tuesday than it is to prevent an accident next Tuesday. I’m outside time. It’s all the same to me. If I go back and stop it, then it never happened. You won’t just “forget” it, it will ever have been.
Person: then why do we live in a world full of suffering here and now?
God: well the plan is to prevent all this from happening at the time of the final reconciliation of all creation.
Person: I thought the final reconciliation was just a consolation for all the bad that has occurred.
God: it’s more complicated than that. It’s not just a consolation, or rectification, it is a complete undoing of all the bad that has occurred present and past as well as future. Not just “forgetting” about it, but actually making sure it didn’t happen at all.
Person: why didn’t you tell us this?
God: I did kind of tell you, but it is not easy for you to understand.
Person: I still don’t get it because the fact is we are suffering here and now.
God: indeed, and it can’t make sense to you how completely I will change reality, past present, and future, for the better, until I show you.
Person: I’m still skeptical how this makes sense.
God: I know.