Input String was not in correct format error

public void bttnCheckIn_Click_1(System.Object sender, System.EventArgs e)
        {
            int children = (int)(Conversion.Val(txtChildren.Text));
            int adult = (int)(Conversion.Val(txtAdults.Text));
            int advance = (int)(Conversion.Val(txtAdvance.Text));
            int discount = (int)(Conversion.Val(lblDiscountID.Text));
            string reserve = "0";
            string remarks = "Checkin";
            string stat = "Active";

            if (lblGuestID.Text == "GuestID" || lblGuestID.Text == null || txtRoomNumber.Text == null || Conversion.Val(System.Convert.ToString(children + adult)) == null || advance == null || discount == null)
            {
                Interaction.MsgBox("Please Fill All Fields", Constants.vbInformation, "Note");
            }
            else
            {
                if (Conversion.Val(System.Convert.ToString(Conversion.Val(txtSubTotal.Text) * 0.5)) > Conversion.Val(txtAdvance.Text))
                {
                    MessageBox.Show("Ops");
                    return;
                }
                string a = System.Convert.ToString(Interaction.MsgBox("Confirm Checkin Transaction?", (int)Constants.vbQuestion + Constants.vbYesNo, "Check In"));
                if (a == Constants.vbYes.ToString())
                {
                    Module1.con.Open();
                    OleDbCommand checkin = new OleDbCommand("INSERT INTO tblTransaction(GuestID,RoomNum,CheckInDate,CheckOutDate,NoOfChild,NoOfAdult,AdvancePayment,DiscountID,Remarks,Status) values (\'" +
                    lblGuestID.Text + "\',\'" +
                    txtRoomNumber.Text + "\',\'" +
                    txtCheckInDate.Text + "\',\'" +
                    dtCheckOutDate.Text + "\',\'" +
                    txtChildren.Text + "\',\'" +
                    txtAdults.Text + "\',\'" +
                    txtAdvance.Text + "\',\'" +
                    lblDiscountID.Text + "\',\'" +
                    remarks + "\',\'" +
                    stat + "\')", Module1.con);
                    checkin.ExecuteNonQuery();

                    OleDbCommand update_guest = new OleDbCommand("UPDATE tblGuest SET Remarks = \'Checkin\' WHERE ID = " + lblGuestID.Text + "", Module1.con);
                    update_guest.ExecuteNonQuery();

                    OleDbCommand update_room = new OleDbCommand("UPDATE tblRoom SET Status = \'Occupied\' WHERE RoomNumber = " + txtRoomNumber.Text + "", Module1.con);
                    update_room.ExecuteNonQuery();

                    if (Conversion.Val(txtSubTotal.Text) < Conversion.Val(txtAdvance.Text) || Conversion.Val(txtSubTotal.Text) == Conversion.Val(txtAdvance.Text))
                    {
                        Interaction.MsgBox("Guest Successfully Checkin! " + "Change: N " + Conversion.Val(System.Convert.ToString(Conversion.Val(txtAdvance.Text) - Conversion.Val(txtSubTotal.Text))).ToString("00.00"), Constants.vbInformation, "Check In");
                        string change = System.Convert.ToString(Interaction.MsgBox("Return change to customer?", (int)Constants.vbQuestion + Constants.vbYesNo, "Change"));
                        if (change == Constants.vbYes.ToString())
                        {
                            OleDbCommand update_trans = new OleDbCommand("UPDATE tblTransaction SET AdvancePayment = " + Conversion.Val(txtSubTotal.Text) + " WHERE TransID = '" + trans_ID.ToString() + "'", Module1.con);
                            update_trans.ExecuteNonQuery();
                        }
                    }
                    else
                    {
                        Interaction.MsgBox("Guest Successfully Checkin!", Constants.vbInformation, "Check In");
                    }

                    clear_text();
                    Module1.con.Close();
                    transID();
                    display_checkin();
                }
            }
        }