Mater Amabilis (a Catholic CM curriculum online) is likewise a lovely resource, free of charge, but so many of the books on it are hard to come by and usually NOT free, that I usually end up using Ambleside instead :/
So I have been working on making some tweaks to Ambleside to make it just a little more Catholic friendly.(
1. Ambleside uses the KJV of the bible, which really does show a lyrical command of the English language. You might choose to read the KJV with apocrypha, but you'll have to remember not all of the viewpoints of the KJV are conversant with Catholic Doctrine. Instead, use the Douay-Rheims version of the Bible, which is a 16th/17th century edition of the Bible, revised (for easier reading) by Bishop Challoner in the 18th century (although that does depend on the version you are reading ;)).
2. Ambleside recommends Trial and Triumph by Richard Hannula for the Church History aspect of Bible study. It is a well written book, that is true, but it is so blatantly anti-Catholic I can't stomach reading any of it to the kids- not even the early chapters! A judicious review of Trial and Triumph will show you that it is a series of small biographical stories of Saints and Martyrs throughout the eras of the church- right up until around the 16th century, when it becomes a listing of reformation characters and founders. For the Catholic, a series of Saintly biographies might be used in its place.
Since Trial and Triump (T&T) is used throughout the first 6 years of Ambleside, I think we have a great deal of leeway in our choices.
Option 1: We use a book of Saint Biographies, not unlike that of T&T, and a book like John Mason Neil's A History of the Church from the Day of Pentecost to the Council of Chalcedon.
Neil's book goes up to A.D. 451, so even though he was Anglican, it is still quite usable by Catholics (and Orthodox) alike :) There is a follow-up book, called Stories of Church History, which might be a good choice to follow on from the original- I have just never seen it OR the Table of Contents, so I cannot say. He does have two other children's books online at google books too :)
Two Beautiful Saint Biographies you might like to use: Saints Lives and Illuminations by Ruth Sanderson and More Saints Lives and Illuminations by Ruth Sanderson
Once Upon a Time Saints by Ethel Pochocki, More Once Upon a Time Saints by Ethel Pochocki and Around the Year Once upon a Time Saints by Ethel Pochocki
The Loyola Treasury of Saints: From the Time of Jesus to the Present Day by David Self (this is not strictly ALL saints, but might be worth getting anyway- it is probably the most like T&T a Catholic book will get).
Patron Saints by Eliza Allen Starr is available free online, and has 20 Saint biographies in it :)
Option 2: You might like to use individual saint biographies, including picture books and chapter books (according to age and preference).
Books by Tomie de Paola you might like to use – some correspond to the Saints below, some are interesting religious books others are religious fables:
Patrick, Patron Saint of Ireland – Saint Patrick, Francis, The Poor Man of Assisi – Saint Francis, The Lady of Guadalupe – Saint Juan Diego, The Holy Twins: Benedict and Scholastica – Saint Benedict (author Katherine Norris), Christopher, the holy giant picture book – Saint Christopher, Pascual and the Kitchen Angels – St. Pascual, Mary the Mother of Jesus, The Miracles of Jesus, The Parables of Jesus, The Legend of the Poinsettia, The Night of Las Posadas.
Books by other authors you might like to use:
Mother Teresa by Demi, The Legend of Saint Nicholas by Demi
St. George and the Dragon by Margaret Hodges, St. Jerome and the Lion by Margaret Hodges, The Legend of Saint Christopher by Margaret Hodges
Brigid's Cloak by Bryce Milligan
The Miracle of Saint Nicholas by Gloria Whelan
Augustine Came to Kent by Barbara Willard
The Vision Book Series is a series of biographies of Catholic personages of interest. These chapter books would be a good choice to read in the place of T&T.
Option 3: You can read through the list of saints below. I have put them in chronological order, and attached a biography (or two or three) - some of which are actually books. All the links are free to read online. There are slightly more than T&T has, but you can leave out a few or add in more to create a more pleasing arrangement of readings. You can also use this list in conjunction with the books in option 2 above :)