Gets the children (K), in logical order: structure elements, marked-content references and object references.
public IReadOnlyList<PdfStructureItem> Children { get; }
ISO 32000-2 Table 355 (K), §14.7.5, §14.8.2.5 (the order of K is the logical reading order). Entries that are not
valid children, and children that would close a cycle or exceed the depth limit, are left out with a diagnostic.