I'm generally for leaving books as they are — original spelling and grammar (American, British, Australian, whatever), vocabulary (jumpers, sweaters, parking lots, car parks), money (dollars of whatever variety, shillings, pounds, in the original quantities), and so on, EXCEPT when it's children's books, in which case I feel spelling and grammar should definitely be regularised to the standard of the region they're published in, and perhaps very confusing vocabulary could be changed (like jumper, which means different things in different varieties of English).
I think older children are quite capable of understanding and actually relishing differences in school systems, candy bars, vocabulary for everyday items, and money systems, depending on location and historical period, and that leaving Famous Five books in the original currency would actually be less confusing than the situation I was in, reading FFs published 70s to 90s and all "updated" to different levels of inflation (and different levels of trouser flare in the illustrations).
But Naughtiest Girl in the School readers are going to be young enough that I think, yes, they need the money to be in a denomination and of a sum that makes sense in context for them.