Fiction; London devastated after bombing, kids go underground and build a community; main character: young girl

I need help finding this book. The cover was of London burning or something, I think. The story follows a young girl who lost her parents and is left with her little brother in a devastated London after bombing, could be the London blitz but I also think it might have something to do with the holocaust, she finds a community of mostly young kids and seems to become a leader. I think the title started with an “e” and I remember the word “firefly” is somehow very relevant. I need to know what book it is, I remember loving it and I had owned a physical copy but I didn’t get to take it with me when I moved in with my dad and stepmom full time. Please someone help me, I’m begging someone please.

Possibly Everland?