Amazon uses collaborative filtering, so people who bought the first book also bought the second one. There’s no rhyme or reason, it’s simple statistics.
Yes, I know. What I am saying is that it seems like an obvious (to me, anyway) way to improve their recomendation system. Unlike adults, kids preferences change fairly predictably over the years. The 7 year old boy who likes Captain Underpants books is likely to be a prime candidate for a Harry Potter book in 5 years. Why not suggest that to him then?