Are there any stats on this? How rare is it that a family nowdays survives on just one income? Maybe I should clarify: a single income in a two-partner family. Also, as a bonus and related to this, how rare is it NOT to have a child in daycare nowadays?
Looking for factual answers, if stats can be found.
This kind of stuff is usually available at the census.gov website. All the data is probably there somewhere, buried in mountains and mountains of strange links and census terms. Ugh. I hate that site, but I have to admit it usually provides results if you stay with it long enough…
Data for married couples:
http://www.census.gov/population/socdemo/hh-fam/cps2004/tabFG1-all.csv
Data for unmarried opposite-sex couples:
http://www.census.gov/population/socdemo/hh-fam/cps2004/tabUC1-all.csv
I couldn’t find any data for same-sex couples.
Source: U.S. Census Bureau, Current Population Survey, 2004 Annual Social and Economic Supplement
And I’m sorry: As I was saying earlier, that site is a horror to navigate and I lost the original page that contained the links to these tables. I don’t know how I ever found the page in the first place 
The table says that there are 59,064,000 married couples in the US.
30,001,000 (50.79%) of those households have both partners employed.
2,034,000 (3.44%) have either the husband or the wife employed, not both.
You can look through the data or the census.gov website for more details.
The Bureau of Labour Statistics (no relation to the census dept, apparently) might be another source of such information, but I’ve done enough digging through government sites for one night, sorry.
Wait… I misinterpreted the data. Please don’t trust my numbers.
I didn’t see that the table was broken down into labor force and non labor force categories (distinct from employed and unemployed). I dunno how to make sense of that, sorry.
It’s also easy to gforget that a lot of households have one or both person retired.
I doubt you’re going to find a reliable pre-digested factoid that says “Of married couples of child-bearing years, X percent have 1 income, Y percent have 2.”
If you did find such a factoid, I’d wonder about the statistical procedure used to produce it. As well, given the political furore surrounding the issue, I’d wonder even more about the motivations of the (often unknown and unknowable) organization which paid for the calculation.
Also, you’d need to check to see if the states distinguish people who work only a few hours a week (10 or less, etc.); most of the SAHPs I know do something to bring in some cash on the side. I work a couple of days a month, but would still say I’m a SAHM. Does my few hundred dollars a year make us a double-income family?
It probably varies hugely according to geography; I know many single-income families (if you don’t count the spare change the SAHPs earn) and relatively few kids in day care, but that’s partly because I live in an area where it’s possible to live like that–or it was, until housing prices tripled a couple of years ago.