This book is surely a product of its time, having been first published in 1959. The structure of a story being told by an elderly nun to two young girls leaves quite a lot to be desired. There are other biographies written for children that are much better, written as straight fact. Because these three fictional creations have been inserted, the biography of the Saint comes off like fiction. There is at least one inaccuracy, which should have been corrected just by reading a map. It doesn't really change the entire work, but it does call other facts into question.
I suppose this book was written for the children of the 50s and 60s, and indeed I remember books such as these. I know the author had a great reputation back then. She hasn't aged well.
If a young reader is interested in learning about St. Frances Cabrini, Cecchina's Dream is much better.