I agree with Mikel 100%. We (me, hubby, and 2 kids aged 2 and 6) have one car, a fairly large estate car with the most fuel-efficient engine we could find (a turbo-charged diesel). We bought it in 2002, planning to keep it as our only car for about 10 years, and then buy another.
We use the car for weekend trips, grocery shopping, etc, but for getting to work/school, we have bikes. I take my daughter to school and son to the childminder using a childback tandem and trailer. You can see photos of us here if you like:
Snow on a School Day
So bike commuting is not just for single people, it works for families as well!