string inputFile = "Notizbuch öffnen.onetoc2";
string dataDir = RunExamples.GetDataDir_NoteBook();

// By default children loading is "lazy".
Notebook notebook = new Notebook(dataDir + inputFile);

foreach (INotebookChildNode notebookChildNode in notebook) {
 // Actual loading of the child document happens only here.
 if (notebookChildNode is Document) {
  // Do something with child document
 }
}