There are some small grammar-punctuation things I'd have pointed out. I do think it should be "our children," or "the toddlers," as "our toddlers" feels to me to break the flow, and I wouldn't go for toddlers anyway.
It is interesting though, that you consider this the greatest sin, and greatest tragedy, not being cast from the Garden, for instance. And the sin wasn't humanity's, but the angels', or demi-humans', back then, which led to the flood. At least to a largest degree.
Besides, the covenant God struck with man after the flood, he said that the rainbow is his mark to remind us he will not repeat it, so you can't blame everyone else for the colourful palette, when God tied the rainbow to the flood as well.
I did not say that I consider it the greatest sin. The Fall does have claim to the greatest tragedy in the spiritual sense - separation from God. "Greatest tragedy" here may be hyperbolic. I was really trying to draw attention to the fact that pretty much every human was killed. It is a disaster worse than pretty much every post-apocalyptic movie and TV show.
Concerning the angels/demi-humans, Genesis 6:5 (NIV): The Lord saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time.
To me, while the angels may have been responsible for great evil, humanity had chosen a path against God.
Your point about the rainbow is good. It is a beautiful symbol for such a grim sort of promise though (it is a positive promise, but a promise not to destroy by flood does feel a bit grim).
Many of the stories in the Bible are difficult to share directly with young kids, so they get lightened a bit. It is understandable. However, I find the story of Noah to have the greatest contrast between the story in the Bible and the story as we present it to kids. That is why I wrote this poem.
For example:
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