The application would be done with one dependant, assuming you told your broker the truth, so yes, it would impact the loan application for sure - babies are expensive. But worth it, most days
The actual amount varies from lender to lender, but they generally add $3-5k+ on per kid to your annual expenses.
Lenders usually assign a $350 liability per month for each dependent.
It's not so much the dependents that put a dampener on borrowing capacity - it's the loss of an income for a period of time that can cause dramas for your borrowing capacity. Having said that - it's also important to not stretch yourself during this period too - it can be stressful enough!