How to obtain a Visa quickly in Hong Kong

Hello, I will be traveling to China from Los Angeles, USA with my adult Son in May. We are flying into Hong Kong for 4 nights and then plan to travel to Shanghai or Xian for 2 nights. We are then going to stay in Beijing for 4 nights before flying back to Los Angeles, USA. It is impossible for us to get our entry permit prior to our arrival into Hong Kong because we are currently US citizens living in Mexico and cannot get to a consulate in our US states of residency. Our flight into Hong Kong arrives at 20:30 on Friday night. Please let us know how we can get our entry permit in a timely manner so we can leave Hong Kong by Tuesday afternoon.

Thank you for your help!
Kathi