Call function of a child form that is inside a panel c #

Good I have this question of how to call a function, I have a form that handles "Panel" (Panels) and it has its buttons inside the main form and in a panel I have a child form, I would like to know how to execute a function of the child form inside the panel

    private void AbrirFormInPanel(object FormHijo)
    {
        if (this.PanelCentral.Controls.Count > 0)
            this.PanelCentral.Controls.RemoveAt(0);
        fh = FormHijo as Form;
        fh.TopLevel = false;
        fh.Dock = DockStyle.Fill;
        this.PanelCentral.Controls.Add(fh);
        this.PanelCentral.Tag = fh;
        fh.Show();
    }

    private void btnUsuarios_Click(object sender, EventArgs e)
    {
        AbrirFormInPanel(new Panel_Usuarios());
    }

    private void btnES_Click(object sender, EventArgs e)
    {
        AbrirFormInPanel(new Panel_Registros());
    }

    private void Principal_Load(object sender, EventArgs e)
    {
        AbrirFormInPanel(new Panel_Registros());
    }

For example, I would like to push the "btnES" button to execute a function that is in the "Panel_Users" form, in case the panel that is open is "Panel_Users"