Charles is head of state, Harry is son of head of state, so immediate family.
“Diplomats and Officials Requiring A-1 Visas - Examples:
-Head of State or Government, regardless of the purpose of travel
-Ambassador, Public Minister, Career Diplomat or Consular Officer coming to serve at a foreign embassy or consulate in the United States, such as an ambassador or consul
- Certain government ministers in positions equivalent to U.S. cabinet level secretaries coming to the United States to perform official, government related duties for not more than 90 days
- European Union (EU) and African Union (AU) delegation representatives
- Immediate family members of an A-1 visa holder”
travel.state.gov