You seam to have read the licensing terms. So apply them.
There is no difference between a children's book and a standard, non-children book. I would classify use in a book as editorial (like) use, so my opinion on this is, that, yes, you need to credit.
The type of licence (extended or standard) is not critical information for this. You would need to credit in both cases. But yes, as of your description, a standard licence appears to be appropriate.
Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788
(Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based on reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).