Thailand passport holders are exempt from a visa to Hong Kong for up to 30 days, you or your Thailand passport don't need a visa to Hong Kong, but both of you need a visa to Mainland China. You two have to prepare your original passports and passport photos, fill the visa application forms, book the round-trip flight tickets and hotel reservation in China, then submit them to the visa officer of China embassy in Thailand, once your application is accepted, you will be given a Receipt, please pay for fees and pick up the visa and passport on the date prescribed on the receipt. You can read China Tour Packages(https://www.travelchinaguide.com/tour/default.htm) to know some information about your Hong Kong and Mainland China trip.