Catchni tu exception a zavolej na ni printStackTrace() a pastni sem vysledek...

try {
    System.out.println(list_of_stackPane.get(0).toString());
    System.out.println(list_of_stackPane.get(0).getChildren().toString());
    list_of_stackPane.get(0).getChildren().add(new Number(10));
} catch (Exception e) {
    System.out.println(e.printStackTrace());
}