example.dart

class ParentWidget extends StatefulWidget {
  const ParentWidget({Key key}) : super(key: key);

  @override
  State createState() => new ParentState();
}

class ParentState extends State<ParentWidget> {
  String childString;
  
  Widget build(BuildContext context) {
    return new Column(children: [
      new Text(childString ?? 'blank'),
      new ChildWidget(updateString: updateString),
    ]);
  }

  void updateString(String value) {
    setState(() {
      childString = value;
    });
  }
}

class ChildWidget extends StatelessWidget {
  ChildWidget({this.updateString});

  final void Function(String) updateString;

  @override
  Widget build(BuildContext context) {
    return new RaisedButton(child: new Text('add text'), onTap: () {
      updateString('foo');
    });
  }
}