Center point

private Point offset;

private void tuaForm_MouseDown(object sender, MouseEventArgs e)
{
    offset = new Point(-e.X, -e.Y);
}

private void tuaForm_MouseMove(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        Point mousePos = Control.MousePosition;
        mousePos.Offset(offset.X, offset.Y);
        this.Location = mousePos;
    }
}

private Point offset;

private void tuaForm_MouseDown(object sender, MouseEventArgs e)
{
    offset = new Point(-e.X, -e.Y);
}

private void tuaForm_MouseMove(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        //Calcola la differenza fra la posizione corrente e quella precedente
        Point diff = e.Location;
        diff.Offset(offset);

        foreach (Control child in Controls)
        {
            //Ottiene la nuova posizione per il controllo
            Point childPos = child.Location;
            childPos.Offset(diff);
            child.Location = childPos;
        }

        //Imposta la nuova posizione di riferimento
        offset = new Point(-e.X, -e.Y);
    }
}