So, not only is this one of the more fucked up and twisted stories that people tell children, it gets more and more twisted and fucked up the farther back you go, until eventually there were no rats and there was no flute or fiddle and the "pied piper" was actually just a fucked up dude who murdered a bunch of kids in this town back in the 1200s and left their carved up corpses in the woods.

So, is the moral of the story "always keep your promises"?

No, the moral is "sometimes you're poor and live in a shitty little German city and then some asshole kills your kid" the end. Except it isn't REALLY the end, because eventually the Brothers Grimm will retell the story of how your kid was murdered and add a moral twist, making the murderer the victim and eventually it'll be a cartoon starring Porky Pig and the murderer will be a character in The Flash comic books and no one even knows that you or your dead kid even existed.