The first book for young people published by Charles Knight for the Society for the Diffusion of Useful Knowledge (SDUK) that I collected was the anonymously authored, The Old Sports of England issued by Charles Knight in 1835. Other than the identification of the work as part of "The Library for the Young" on its half-title, the work was printed in larger type than most or all of the books in small format for the SDUK, and contained a profusion of full-page woodcuts.
The second work for young people that I obtained published by Knight for the SDUK was Pictures and Descriptions of Remarkable Events in the History of England with one hundred and twenty-six woodcuts after designs by W. Harvey, and other illustrations. This work was issued in two volumes in 1838. Apart from having many illustrations, this work was typeset in substantially larger type than other works Knight published for the SDUK, with the intention, one assumes, of making it easier to read. It was not specifically identified as part of the "Library for the Young," perhaps because Knight expected the book to appeal to both the youth and adult markets.
Based upon my collecting of Charles Knight publications for around ten years, it appears that Charles Knight did not publish many books for young people, and that the books he published for youth were either issued in smaller editions, or had a lower survival rate than some of his other books, since they tend to be harder to find today.