Actually, I believe that the readers are not meant to figure it out - rather, we are meant to be believe that if we did learn it, we would actually be able to travel into these other universes.
In An [sic] portrait of Tonke Dragt (the link is to a PDF of an English translation), we are told:
Tonke is never satisfied with ordinary reality. She enjoys blending reality with fantasy,
and does it so well that her readers sometimes become confused. Every day she receives letters from children and adults who want to know the word that might lead them to another world
The summary of the book by The Foundation for the Production and Translation of Dutch Literature (NLPVF) says:
The exciting literary thought-experiment upon which the story hinges is the idea that other worlds might exist: mirrored worlds whose different time dimensions intersect every leap year between 29 February and 1 April, creating a moment when it is possible to step from one world into another, as long as the right word, which Dragt keeps secret, is spoken.
I believe rather than "keeps secret", a phrase like "conceals within the story" would be used if the reader (especially a child) was really meant to seek it out.
I don't know of any evidence that suggests that the word is hidden inside the book somewhere.