Heh... another comment that's just occurred to me:

Here, most people would also say no - they'd want the "bonus" for children to be equal for low- and high-income families, but they do not want the "penalty" for lacking children to be the high for same and low income.

Again, this is by no means necessarily irrational. The effects of government policies are by no means limited to their immediate fiscal implications. People typically care much more -- and often with good reason -- about their status-signaling implications. By deciding to frame its tax policy in terms of "X is good and normal, but Y is even better" rather than "Y is good and normal, but X should be penalized," the government sends off different tremendously powerful signals about the status that it ascribes to different groups of people.

Average folks can be terribly innumerate when asked questions of this sort, but they'll clue onto the status implications of different alternatives instinctively. These concerns may well be important in practice -- even if a myopic view focused solely on the accounting issues would dismiss them as sheer bias. Of course, it's arguable to what extent this particular example is about realistic status-related concerns, but that's a question to be answered with non-trivial reasoning, not outright dismissal.