如何在flutter中访问Firebase实时数据库快照上的子属性？

import 'package:firebase_database/firebase_database.dart';

final DatabaseReference databaseReference = FirebaseDatabase.instance.reference();

databaseReference.once().then((DataSnapshot snapshot) {
  // 访问子属性
  var childValue = snapshot.value['childKey'];
  print(childValue);
});

databaseReference.onValue.listen((Event event) {
  var snapshot = event.snapshot;
  // 访问子属性
  var childValue = snapshot.value['childKey'];
  print(childValue);
});