I was in a similar situation. After many telephone calls one of the staff advised me to say that I am not able to return to work as I cannot pay the nursery fees and to ask for advanced FSF. They did not give it to me at the end but I guess it depends on the person that is dealing with your query? I returned to the same work as you
Have you considered a childminder? They’re more likely to expect payment at the end of weeks/months than upfront?
No advice but right there with you, we've got his first 2 months worth of nursery Bill before I've been paid, don't even get universal credit. Can you talk to the nursery and ask if they can wait a little while longer if you are due pay soon or have been fairly understanding