Going a little back to the original topic, I think the Govt chooses to "subsidise" raising children with their eyes fixed on the big picture rather than just easy pandering to a specific voting demographic.
I'm guessing Australia was in need of a boost to its birth rate to maintain viability to its population and in particular to it's working population. The trend towards ageing populations in all the developing nations is quite of concern. The foremost govt concern would be the shrinking tax base and burgeoning number of recipients for welfare and govt support. As the proportion of the population retired and placing increasing burden on public services such as healthcare increases there will be a growing need to spend tax revenue. If this is coupled with a shrinking number of workers/tax payers; then its not hard to see what will happen in a generation or two.
In light of this sort of scenarios, I would think the Govt would consider a few thousand dollar incentives to be a small investment for the long term to create more taxpayers.
Kenny