I would like some guidence (Hong Kong)

My Girlfriend has applied for study in HK university. She wants me and our child to go with her if she is admitted. I am just worried as she is chinese and i am not.
1) Will i get the long term stay as HongKong belong to china now?
2) Is it possiable i would be able to do a job legally or what would be the legal way?
3) How much the cost of living should we consider (family of 3)?
I am open to any suggestions, options. I would love if they not guess, most likly kind of answers.