What pointer lifetime to use when iterating through a tree?

struct BKTree:
    var root: Optional[BKTreeNode]

struct BKTreeNode(TestableCollectionElement):
    var text: String
    var parent_distance: Int
    var children: List[BKTreeNode]

nodes_to_process = List[UnsafePointer[BKTreeNode]](
    UnsafePointer.address_of(self.root.value())
)

while len(nodes_to_process) > 0:
  current_node_ref = nodes_to_process.pop()
  current_node_distance_to_query = levenshtein_distance(
      current_node_ref[].text, query
  )

  for child in current_node_ref[].children:
      if (within_distance):
          nodes_to_process.append(UnsafePointer.address_of(child[]))

nodes_to_process = List(Pointer.address_of(self.root.value()))