Literal belief in the flood story

[Esquilax]

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).

So what evil thoughts do babies think? I mean, a child isn't even self aware until like two, so your blanket, stupid generalization is just factually wrong to begin with...