What's the name of a book where some kids go to a different world where an Aztec-like culture sacrifice people?

It may be a young adult book. At one point in the book, they trade a pocket knife to some locals who are amazed by the concept, and trade them for one of their knifes, which is much sharper and better, but doesn't fold. The Aztec culture is cutting out the hearts of their defeated enemies who have given up and are in a long line walking up the steps of a pyramid. Near the end a God-like being emerges and i don't remember anything else. Please help, was a favorite of mine when i was very young but can't seem to recall the name.