Use Singleton pattern on Child Form and change the method of invocation in Parent form.

Child form:

//Used for singleton pattern
static frmChildForm childForm;
public static ChildForm GetInstance
{
          if (childForm == null) 
          childForm = new frmChildForm;
          return childForm;
}

Parent MDI Form:

frmChildForm childForm = frmChildForm.GetInstance();
childForm.MdiParent = this;
childForm.Show();
childForm.BringToFront();