Eu finalmente encontrei a resposta para minha própria pergunta. O romance em questão é intitulado Panda Ray .
Christopher Zimmerman sure looks like a normal ten-year-old (even if he is the shortest member of his class). He collects pebbles and has a dog named Buzz. He also tells his whole fifth-grade class at Lincoln Elementary about traveling into the past to see dinosaurs and trilobites. For Christopher - in spite of his ordinary looks - is actually the youngest child in a family of aliens... or are they mutants? (And really, does it matter which?). When Mother hears of Christopher's boasting, she rules as she must: Christopher will be scooped out. That's right - no more omicron for him, no more upsilon. Christopher would rather die than be scooped out, so he enlists Gramps's help and flees on a journey through space, time, and other less recognizable things, and into a classic science fiction adventure
Este site menciona a viagem no tempo através do papel higiênico:
Os comentários parecem ser incrivelmente misturados, com números iguais de pessoas proclamando que é o melhor ou o pior livro de ficção científica que eles já leram.This science fiction novel is about a dysfunctional family of superbeings (aliens? mutants? humans from the future?) in modern America. It reminds me a bit of the writings of Stanislaw Lem, which is not very surprising since the author is probably best known for his English translations of Lem's work. Of interest to this website is the role played by Penrose Tilings and the Golden Ratio in the grandfather's ability to travel in time and space as well as to other universes. The non-periodic tiling built of kites and darts is described in great detail. In fact, we learn how to construct the golden ratio using a compass and straightedge and how the grandfather is able to replicate the tiling using nothing other than toilet paper and liquid soap! Unfortunately, the name "Penrose" is never mentioned, so someone reading this book who does not already know about this mathematical construction will have difficulty finding out more from other sources.