[T]he siblings said in interviews, they were already at work trying to figure out how to manage a gift from their father valued at about $1 billion each that will go to their own charitable foundations. That will propel them, along with a larger foundation named for their late mother, into the top ranks of philanthropy — dwarfed, to be sure, by the Bill and Melinda Gates Foundation and the $60 billion or more endowment from the Gates and Buffett fortunes. The younger Buffetts, given their current interests, could emerge as powerful forces in the areas of early childhood education,...