How to kill newly created process (explorer) after I start one

Question

My program (call it main program) runs under system privilege, and I need to start new program (e.g. pdf reader, call it target program) under user privilege from it. I find this solution: How to start a new process without administrator privileges from a process with administrator privileges? Please check Eugene's answer. By this way, it uses explorer.exe as a bridge to invoke the target program so that the target can run under user privilege.

But I find one issue of it. Each time I run the program, it will invoke one explorer.exe process. And if I run it then kill/close it for multiple times, then multiple explorer.exe processes shown in Task Manager.

One idea to solve it is when I start the target program via new Process(), I can remember its process id and then when quit program I just kill it by its process id. But my code does not work.

The arg path for StartProcess() is string "C:\\Windows\\System32\\notepad.exe"

  public void StartProcess(string path)
    {
        newProc = new Process
        {
            StartInfo = new ProcessStartInfo
            {
                FileName = "explorer.exe",
                Arguments = path,
                UseShellExecute = true,
                Verb = "runas",
                WindowStyle = ProcessWindowStyle.Normal
            }
        };
        _ = newProc.Start();

        Debug.WriteLine($"start my process (id = {newProc.Id})");
    }

    private void KillProcess(Process p)
    {
        if (!p.HasExited)
        {
            p.Kill();
            Debug.WriteLine($"kill my process (id = {p.Id})");
        }
        else
        {
            Debug.WriteLine($"my process had quit already (id = {p.Id})");
        }
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        KillProcess(newProc);
        ShowExistingProcess();
    }

You may find my demo project from GitHub: https://github.com/tomxue/KillExplorerDemo.git