Work Visa Questions

I am going to China to work for a Chinese firm. I am applying for a Z visa for myslelf, my wife and step son. After receiving the visa, is there a time frame I need to enter China before it expires? My wife and I were planning on entering China first, get settled (getting residency permit)and than bring our son into the country about two months later. Does my whole family need to enter China at the same time for the first time? or can we bring my son there later?