You are right in that there is a cost associated with working outside the home, if one has children and I've heard others make the same calculation, but usually with pre-school aged children. Some of your costs depend on living conditions. The second car is usually always needed unless the breadwinner commutes via public transportation, etc.
I would have to say that's not necessarily true. Mr. Ex takes our minivan while I stay home with the kiddos (all 5 and under, down to three weeks old.) He asks me every day if I need the van, and if so, I simply use it and then have him take us home on his break from work. Of course, it helps immensely that we only live 6 minutes from his place of business! :) But we've gotten by with only one vehicle before, and while we were BOTH working, no less. It just takes some flexibility and creativity. Plus, if you were as poor as we were, that's pretty motivating, lol.