It would probably depend on the girl.

plot

The Hidden Treasure of Glaston is an exciting mystery thriller about a boy's journey in becoming a man and his struggle to obtain the Holy Grail that Jesus used at the Last Supper.  The Hidden Treasure of Glaston takes place in Britain in 1171, and the story is seen through the eyes of the main character Hugh, a sickly boy whose father abandons him at Glastonbury Abbey when he is 12 years old. He seeks occupation at the monastery's scriptorium, where it is the monks' responsibility to copy the scriptures as the printing press was not yet invented. Hugh does not show much promise upon his arrival at the monastery at first but makes immense changes as he transforms into a responsible, persevering, and religious young man. It is nearing the end of his journey that he shows the qualities and personal growth that make him seem more refined in all aspects of his personality. He shows little regard towards his well-being as he begins to travel knowing the dangers he would face. The journey unravels with him escaping from a mob that chases after him, finding his way through deserts and caves and making it to Glaston before he gets caught. Despite all the difficulties he faces throughout the story, he shows perseverance. He becomes a new person through his adventures, having developed more strength, faith and courage.

It seems more for a boy. I have already picked out some books (so many to choose from) for the ones below the age of 8 but for 11/12 it is more difficult.

What about a heroine like one of the saints.

https://stritabooks.com/abante/the-queens-blessing
Warning. aussie site. See if you can find it without costly postage

The Wopsy the Guardian Angel books are great. I got ours from St. Jerome Library. Father Gerald or Gerard Brennan’s Angel Food books are also very good. A bit dated but filled with good, short and sweet stories.

Good books that are not expressly Catholic but are not anti-Catholic can be found on the Bethlehem Books website. A great, fun mystery series for the whole family is the Happy Hollister series. For the older boy, a great author is Arthur Catherall. His books are quite suspenseful, which my 10year old boy loves and clean. A lot of them are great coming-of-age stories where a young man is given a situation where he has to prove himself. Again, not expressly Catholic, but nothing immoral that we’ve come across so far. The author traveled a lot when he was in the military, particularly India, so a good amount are set there, which also means the paganism of the area is incorporated, but he makes it clear it is just their superstitious beliefs. We use it as an oppportunity to discuss their beliefs but you could also just skip those ones. 

I find that there's a bit gap between books for younger children (<10 years old) and books for adults ... just not much out there for kids between 10-15 or so.

catholicteenbooks.com

Although these are not books, they are appropriate for children.

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