... fact of history being a series of disastrous snafus and brutal devastation punctuated by brief shining moments of progress without scaring the little kids too much?
-Penguin
I think you're being overly pessimistic here. Every day all across the globe billions of good people wake up, and go about there business in an honest manner, helping others and making positive contributions to society. Hardly any of these people will kill somebody, or rob somebody, or participate in genocide, or plot some evil scheme in their heart. These people and their everyday actions, which are the vast majority never make it into the history books.
The history books are filled with the minority, the very fact something is in a history book is because they are events or people which fall outside of the norm. The history books are never going to talk about the average everyday Joe or Jose or Mohamed, of which at least 95% are decent good people who live their whole life in a positive manner and therefore die in general anonymity.
Out of all the people you've met in your lifetime what percentage of them do you suppose are murderers, or thieves or scam artists, vs how many are honest, good people. Sure we may disagree on politics or religion, but 90% of the world strives to follow same basic code of morality. It's the other 10% that usually makes the history books.
If you only read history, only look at those great and terrible events, then you could be forgiven for thinking the world is a terrible cesspool of a place. But for those that actually go out and life in and experience the world, it is the billions of little everyday things which taken together make up the real picture.
Of course we need to teach kids history, and never sugar-coat it. But kids should also learn in the home and everyday life the other side of the story, that is how to live positively, and most do.