如何访问父类型的ArrayList中的子方法？

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Parent> list = new ArrayList<>();
        list.add(new Child());

        // 访问父类型的ArrayList中的子方法
        Child child = (Child) list.get(0);
        child.childMethod();
    }
}

class Parent {
    public void parentMethod() {
        System.out.println("This is the parent method.");
    }
}

class Child extends Parent {
    public void childMethod() {
        System.out.println("This is the child method.");
    }
}