Hong Kong is completely separate from China for Visa purposes.
This is an Employment Visa, sponsored by his employer?
If so, and particularly if he's on an internal company transfer at a moderately senior level, then it should be pretty straightforward. It will take 4-6 weeks after the application is submitted before you get the Visa. You can also apply simultaneously for your and your kids' Dependant Visas. Once you have the visas you will get HK ID cards which then make getting an apartment lease (assuming the company isn't doing that in their name), bank account, and so on, easier.