Children’s book done in only Spanish

First of all, the 2015 "Provisional Guidance for Transcribing Foreign Language Material in UEB" has been replaced by 2022's "Guidelines for Braille Transcription of Languages Other Than English", which you can find here: https://brailleauthority.org/guidelines-braille-transcription-languages-other-english-approved-may-2022 .

Method 3 corresponds to Category 2 under the new guidelines.  Under this category, it states: "This category also includes literature entirely in a non-English language for use in North America, such as advanced textbooks or literature in a language other than English." Your project fits under this description.  In this Category, Spanish text is transcribed uncontracted, using the Spanish braille code's symbols for accented letters.  Punctuation, indicators, and other symbols are transcribed in UEB.  T-pages are transcribed in UEB, and Spanish titles/names/etc. that appear on the title page are transcribed as Category 1 text (using UEB modifiers and contractions).  (The title on braille page 1 and the running head are transcribed as Category 2 if they contain non-English words.)

If you have a client who requests a specific type of transcription, you can use a different Category, but without that request Category 2 is the appropriate way to go for this type of text.  I do recommend reading through the new guidelines, because in some areas they are much clearer than the former document.  You can also go the Webinars Archive on this website (under Resources: Webinars) to find the Foreign Language webinar "Overview of BANA’s Guidelines for Braille Transcription of Languages Other Than English" from 2023 for more discussion on the change of guidelines.