Each area may have a field parent which contains either null or the ID of another area. There should be an easy way to get that information via the endpoints.
The admin interface should allow to set an area's parents. There's no need to have a publicly available endpoint, as no user can modify it.
The URL scheme for areas should not change; say we have the areas A and B, where B has A as a parent. Both of them are still available via /areas/a and /areas/b.