Mine was the same. You can submit your PR application first then raise webform when there is a Birth Cert.
Note: my application was approved and got P2.
You can submit your PR application first. Once you get birth certificate then raise a webform. It does not matter when submit webform without AOR.
Once your application is acknowledge. The officer will request additional document "Additional Family Info IMM5406" for you and your spouse. All you need to do is mention your child and submit document.