I'm looking for the title of this book I read, at least 10 years ago. It had a siblings escaping London during WW2 and instead of reaching the countryside they enter this odd alternate reality. In this reality the people there decide to take care of these kids, and they have some sort of credit belt system.
I remember a few specific details, one of their new friends loves butter pies, and steals one of the characters credits, and then gets pretty sick when he eats his way through dozens of them.
The plot revolved around some sort of caskets, arranged in some sort of order, silver, gold, and lead. I don't entirely recall what happened, but they locate these caskets? or egg things? but near the end there is a scene where the bad guy who has been collecting these devices is walking towards a big clock, and the kids are trying to restrain him somehow. If he succeeds then the city will be destroyed etc. Obviously the kids save the day. I don't remember if they make it back to their own universe afterwards. I believe it was a standalone book