PROVIDENCE, R.I. — While many nonprofits and businesses have struggled during the pandemic, Robert F. Kennedy Jr.’s anti-vaccine group has thrived. An investigation by The Associated Press finds that Children’s Health Defense has raked in funding and followers as Kennedy used his star power as a member of one of America’s most famous families to open doors, raise money and lend his group credibility. Filings with charity regulators show revenue more than doubled in 2020, to $6.8 million.