i'm visiting my son in HK and going to China from there

1. Ticket from HK to China is not necessary as you may easy to go to Shenzhen from HK by ferry.
2. Do you mean the application form when your fill? if yes, all members should be included if they will visit China together, if children share the same passport with their parents, Visa application form V2011B is also required.
3. The photo should be recent, colored, front view, in 48mm x33mm size without head covering. Background is not specially required. Previous visa is required if you had visited China before.