After wrapping her arms around the retail giant Wal-Mart and trying to cajole food makers into producing nutrition labels that are easier to understand, Michelle Obama, a healthy-eating advocate, has her sights set on a new target: the nation’s restaurants. A team of advisers to Mrs. Obama has been holding private talks over the past year with the National Restaurant Association, a trade group, in a bid to get restaurants to adopt her goals of smaller portions and children’s meals that include healthy offerings like carrots, apple slices and milk instead of French fries and soda, according to White House...