Gets the item's children, in the order of their First/Next chain; empty for an item the outline no longer reaches.
public IReadOnlyList<PdfOutlineItem> Children { get; }
ISO 32000-2 §12.3.3: "the items at a given level shall appear in the order in which they occur in the linked list". The children as the links are now; a list read earlier is not updated, read the property again.