@AIR Hi, I have had this conversation in a different thread but can't remember where exactly.
Actually, it is subject to interpretation. It can be that for example, you are the main
applicant for your 489 and your partner is the dependent, then for 887, your partner can be the main applicant and you can be the dependent but I will say your example can also be possible. Anyway, the visa processing fee for 887 is cheap so if by the time your are eligible for application and your partner is not, and if you can apply on your own then go for it then your partner can apply on his/her own.
Please note that if your partner will apply (or can apply) for his/her own 887, he/she needs to satisfy both 2yr stay and 1yr work. As oppose to if your partner will be your dependent, he/she only needs to provide evidence of 2yrs stay.
I don't see any issue once you have kid(s) coz you can add newborn kids to any existing application prior to a decision.