If we're still talking about Australia, there is no Family Tax Benefit for a single income provider earning above $104,281. It tapers off as you approach that.
As a general rule the Australian Government make it very difficult for families to have a stay-at-home carer. It's substantially more viable (monetarily speaking) for most families to send their kids to daycare, which is Government supported, and both work.
The married, childless middle class really does get a bit screwed when it comes to taxes.