I think that you are going to have to make a critical choice in terms of how this is going to be written. From my own view, it seems to me easier to make a case as to why Serraillier's work is a good one as opposed to a bad one. If you don't have a specific basis as to why The Silver Sword is lacking in some capacity, then it seems to make more sense to go for the analysis that it is a good book and find reasons to substantiate it as such.
I think that one of the reasons The Silver Sword is a good book is that it shows how survival is only dependent upon community. The only way to withstand the Nazis is through trust and dependence on one another. For example, Jan and Joseph have to work together and the children cannot simply leave one another. The only way to increase survival is for individuals to work together, regardless of overwhelmingly cruel conditions. In this idea, the novel represents a sense of intellectual goodness and this can be seen as a quality of a good book.
I think that another reason it can be seen as a good book is because it was one of the first books to explain the horror of World War II that the Nazis inflicted on millions. Over time we have come to understand the extent of the Nazi damage. However, in the mid 1950s, the world was only emerging to the horrors of the Nazis in World War II. The Silver Sword is a good book because it raises awareness of such a condition. At a time when so many were seeking to simply move on from World War II, the book is a reminder of what actually happened to millions of people. The book left an impression on author John Boyne, whose own book, The Boy in the Striped Pajamas, was quite effective in depicting Nazi brutality:
...the book stands out for me as a great children's classic – [it] was my first introduction to the Second World War in fiction, to the horrors of the Nazi era, and the fear that capture could instill in the minds of its young heroes Ruth, Edek and Bronia.
The ability to recreate the terror of the Nazis is another reason the book can be considered to be a good one.