Literal belief in the flood story

[Minimalist]

The problem is: the children. The whole notion is God was mad at the wicked people, so he killed them and their kids to make things right. Now, there's no way that the children who were sufficiently young would have been wicked, so why did he kill them?

The error in your argument/understanding lies in the above quote. Genesis 6:5 states: "And God saw that the wickedness of man was great in the earth and that every imagination of the thoughts of his heart was only evil continually." This is the reason the flood was sent, to judge mankind who's hearts were only evil continually. Your statement claims that children would not have been wicked. God states that everyone on earth was evil continually.
If you assume the account of the flood to be true, which you have to make your argument, then you must assume that everyone on earth, all the imagination of the thoughts of their hearts was only evil continually (as it is part of the account).
Today there is a door much like the door to the ark. Walking through this door, like the ark, leads to salvation, an escape from the coming judgement. This door, like the ark, is a narrow door. This door is Jesus Christ.

And here you have the evidence which proves the OP correct.

This myth is stupidly contrived and terrible. When people accept it as true, they make some of the most creepy, and morally bankrupt excuses for God I have ever heard.

Xtians are creepy and morally bankrupt!