children Property should work for all Parent

children Property does not always include all direct descendents.
At least SplitPane "children" are in items Property.

This makes almost impossible to uniformly walk the widget tree.
In particular the following snippet will NOT work if SplitPanes are involved (I'm not sure about other kind of containers).

static class Visitor {
    public void visit(Node node){
        ...
    }
}

protected void walkWidgets(Node n, Visitor cb) {
    if (n instanceof Parent) {
        Parent p = (Parent) n;
        for (Node c : p.getChildrenUnmodifiable()) {
            walkWidgets(c, cb);
        }
    }
    cb.visit(n);
}