Hi Bro, congratulations for new born.
I understand that your baby born after you applied for 887 visa. So for that you just have to inform change in circumstances form which if form 1022 along with passport and BC. You have to just update in your immi account. And when your co allocate he will give 489 Visa and bringing 887 visa first and later asked for medical examination.
In case if your baby born before applying 887 visa, you have to fill form 1436 and all which is I have done because my daughter born right before I applied 887.and it's takes longer time to process your file. I paid around 98$ to add my daughter in 887 visa.
In your case is very simple. No any extra cost to add newborn . Hope this clear to you.