To get him there and working as soon as possible, some sort of temporary work visa - see here.
http://www.immigration.govt.nz/migra.../requirements/ (Which one will depend on exactly the circumstances of the employer and the job offer, which you should be able to see for yourselves, and talking to the employer.) Along with this you can apply for a partner-sponsored temporary work visa for yourself
http://www.immigration.govt.nz/migra...milystream.htm (and no, you don't have to work straight away - it just gives you the right to take any job when you're ready). Your five-year-old will need a student visa.
http://www.immigration.govt.nz/migrant/stream/study/ (That form filled in as if by her.) The baby will need a visitor's visa.
http://www.immigration.govt.nz/migrant/stream/visit/ You can apply for all those together, with a covering letter to link them. The official will supply details like your husband's visa number as it becomes available.
That's the basic, to start you off. You can also apply for Residence at the same time, for which all of you go on the same one application.