I don't think it's been said yet, OP well done for passing the interview and getting the job.
I employ and interview regularly, and I'd like to think in this situation, considering it's 0 hours anyway and you're being taken on as part of a cohort and not for a set individual role, I'd actually just bank your CV for the next cohort after you became available and consider it one less interview to conduct in a few months time.
A lot of people here have absolutely no idea how bank work/ agency work operates but feel compelled to tell you how awful you are anyway 
For those wondering: Banks (im assuming you're in healthcare, op?) are a rolling conveyor belt of workers. There's a massively high turnover and because of that you literally hire and interview constantly. The whole point of banks is you fill in gaps in other workforces and you have a number of staff on your books to do that, because the nature of 0 hours means workers pick and choose their hours to suit themselves. It's like job sharing between sometimes hundreds of people.
Yes, if this was a specific role the company were hiring for then OP would have done something really shady and although legal, would have put some noses out of joint. However, I actually think she's made a good call here and the job she's applied for is built to fit around people like OP who have mega childcare responsibilities.
OP, don't offer voluntary work, you don't need to. Just tell them your situation and express that you'll be available to work from x date when you'll begin to pick up shifts, alternatively you can offer to do any training days they want you to do on the next lot they've booked if it's in the next couple of weeks, or offer to do the training days unpaid after your first 6 weeks of Mat leave.
Congrats and good luck with both the pregnancy and the job!