i don't get how people who work 0hr contracts and shift work find childcare if they have to block book it when they have no idea of what they would be working
It's the latter, generally.
You have to remember that a childminder needs a guaranteed income of their own. If you say that this week I'll use you for 5 days, but next week I only need you for 2, that means the childminder has 3 "empty" days when she is not getting paid. It's frustrating as the parent trying to arrange childcare but you have to remember these are all business, and very few childminders are in a position to have so many empty spaces during a week.
As I mentioned before, we were organising childcare around shifts. We agreed with our childminder that we basically reserved a space 5 days a week, but paid for a fixed number of hours care every month, say £600, for 100 hours care a month at £6ph. We were then able to tell them a few weeks in advance what hours we would need during a particular week. If we needed more than 100 hours care a month, they billed us for the extra cost. If we used less than 100 hours care, though, we certainly didn't get a refund. They needed their guaranteed income every month, and to be honest that was a great deal we cut because it was cheaper for us than reserving a full-time place.
It's frustrating working it out, but that's just how it is. Gets easier when they hit 3 and you get funded hours to cut your costs.