public class Form1 : System.Windows.Forms.Form
{
    //中略

    private void button1_Click(object sender, System.EventArgs e)
    {
        Form f;
        f = new Form();
        //f = new EditForm();

        f.Move += new EventHandler(childForm_Move);

        f.ShowDialog(this);
    }

    private void childForm_Move(object sender, EventArgs e)
    {
        Form f = (Form)sender;

        int x = f.Location.X - 100;
        int y = f.Location.Y - 100;
        this.Location = new Point(x, y);
    }
}