I remember reading that. It was 30 years ago so I may be remembering it wrong... We had this as a assigned reading in school but only the first chapter or something. It was in our textbook kind of as a short story. So we read only the part about the kid dying without knowing the story continues. I later found the entire book somewhere and kept reading and hey, the brothers met again in the magic world. I don't remember them dying at the end or anything else for that matter. It was one of the least traumatizing books they assigned us. In most Polish books and novels some kid dies, or some animal dies, or there's war, or concentration camps or some other torture.