一道考察 JavaScript 的 Class 知识点的题目，难倒我了，求解

现有一段代码如下：

class Parent{
   say(){
      console.log('that method called from parent method');
   }
}

class Son extends Parent{
   sayChild(){
      console.log('that method called from son method');
   }
}
Son son = new Son();
son.say();
son.sayChild();

假设现在你不可以使用 extends 关键字来直接继承父类，只给你一个对象，你应该如何完成以下代码块：

class Parent{
   say(){
      console.log('that method called from parent method');
   }
}


var sonObj = {
   sayChild(){
      console.log('that method called from son method');
   }
}

function getSonClassByExtends(sonObj,Parent){
	// finish the function code and make it work
}

var Son = getSonClassByExtends(sonObj,Parent);
Son son = new Son();
son.say();
son.sayChild();

各位前端大佬，这道题应该如何求解，谢谢！