Buffett's premise on not giving much of his wealth to his children, goes back perhaps 40 or 50 years now based on what he has said. According to Gates, an article that Buffett wrote for a business magazine, published ~25 some years ago now, informed and altered Bill's thinking on the detriment of giving such immense wealth to the next generation.
The Giving Pledge is primarily for other billionaires to pledge to give away at least 50%. The 50% level made it an easier bar to jump over, as of course not all of the billionaires want to match the 99% marker Gates & Buffett have set.