Our nanny told us upfront that her rate was $3 for each additional child. Perhaps your nanny already has a rate? Not that you can't negotiate. But she may have expectations, too.
Yeah, find out if she has this attitude problem so then you can go find a great nanny for the 2-kid market rate of $17-18/hr.
Yeah, all those "great" nannies who only command 17-18 an hour. They really are great, OP, and could make so much more but they just don't want to.
To be frank, it doesn't matter what a nanny wants to make. The market drives the rate and that is comprised of the job details, speciality skills and supply and demand.
Everyone wants to make $25/hr for one child and no extra work. But unless you find a family unaware of the market or you are extraordinary, you will find it difficult to find such a job and could lose out working for an amazing family who pays market rates. For families, it is important to remember that choosing to overpay a nanny does not guarantee a better nanny.