On a perfect Southern California morning not long ago, a gaggle of children gathered in the backyard of a million-dollar home in an upscale Los Angeles neighborhood to celebrate the birthday of twin four-year-old girls. The host parents had rented a petting zoo for the day, and kids jumped gleefully in a bouncy castle out in the driveway. On the terrace, a few parents chatted beside an alluring spread of bagels, coffee and fruit.