// 设置父窗体为Mdi容器
this.IsMdiContainer = true;
// 创建子窗体实例
ChildForm frm = new ChildForm ();
// 设置子窗体的Mdi父窗体对象
frm.MdiParent = this;
// 打开子窗体
frm.Show();
//关闭子窗体
this.MdiChildren[0].Close();

------解决方案--------------------
问题2：

C# code
private void timer_Tick(object sender, EventArgs e)
        {
            this.Close();
        }

      // 
        private void Form_Load(object sender, EventArgs e)
        {
            Timer timer = new Timer();
            timer.Interval = 30000;
            timer.Tick += new EventHandler(timer_Tick);
        }