Do we need an S2 Visa to visit my son in Hangzhou and would that cover a 3 night stay in Hong Kong?

Hi,
I am going to visit my son, who is teaching in Hangzhou, for a week in March with my daughter. Do we need to apply for a S2 type and get a letter of invitation from him, along with a copy of his passport, residency permit, his birth certificate and ours?
We are only going for a week and as its his birthday while we are there we wanted to take him to Hong Kong for 3 nights. Do we need a special entry permit to do this and will my son have to apply for any kind of entry permit in China?
Regards
Angie