Since I started the money worry (sorry!) I thought I'd update a few things and add on to Shoe's helpful post.
We should all qualify for either some working tax credits www.gov.uk/working-tax-credit/eligibility or child tax credit www.gov.uk/child-tax-credit/overview You won't get both. You also have to be very careful when you start working again as they are NOTORIOUS for overpaying for a year (even after you've told them about your income and they've told you that you still qualify) and then demanding it back.
As soon as you earn over £30 - 34k you lose it (both!) It's a painful hit.
If you rent, depending on your area you will probably qualify for Housing Benefit - this has been the biggest help for me. Mostly because I live in insanely overpriced London but also because as soon as I went over £32K I lost the incredibly helpful tax credits.
Child benefit is about £80 a month. And you can get Council Tax single person reduction too.
Child care is obviously the killer - under £50 a day is amazing. Here in London they regularly hit £90/day for a baby still in nappies. £70/day is the non-nappy rate lol. There are definitely some for less but in most cases you get what you pay for.
The other thing to note (and what I wasn't expecting) is that even if you get your 15 hours a week from age 2 (we didn't get the 30 hours when my daughter was that little but I expect it's the same) most nurseries won't let you use just your 15 hours. You'll usually have to use say 3 full 7 hour days (or equivalent half days) which they'll apply your 15 hours to and then you pay the difference. If you use a council nursery this is usually not the case but was in 99% of the cases of people I know who used nurseries.
I'll be honest I was almost better off on benefits than I was working. I think when I started back in full time work, once I took childcare, commuting and work related bits (clothes etc) into account the "extra" from working was like £165/month.
Another lifesaver is to join/start a babysitting circle where people sit for each other in exchange for babysitting (i.e. I watch yours/you watch mine) Absolutely brilliant for those of us who don't have loads of family near by and would occasionally like to leave the house by ourselves!
You can totally do it. You can never plan for the "right" time to have a baby. When I was married my ExH and I planned everything - bought the forever house, had good jobs, two cars and 3 months into the pregnancy the market crashed and he lost his job and didn't find another for 11 months. Lesson learnt!