Hong Kong does not require entry visa for my citizenship.

My first destination with the family is Hong Kong staying 4 days and then visiting to Guangzhou by train. I have already a valid single entry China visa I will spend some days in Guangzhou and coming back to Hong Kong for departure through HKIA.
1- Is there any problem to return back to Hong Kong?
2- What is the name of train stations in Hong Kong and Guangzhou to travel by.
3- Is it easy to travel by train with three kids and 3 pieces of luggage, the luggage will be stored in special cabinet or can carry them with us inside the passenger cabinet?
4- Where is to check the passports?
5- How can I reach directly from train station to the airport when I arrive to Hong Kong how will be my routes?
Thank you