Spritz won't work for books with illustrations. It's a complete no-go for photo-books and children's books, for example.
By the way, there is another speed-reading aid technique which can fit children books - beeline (gradient-coluored text). But I'm not sure how can it affect children learning to read.
Have you seen this two books?:
https://bubblin.io/book/abcd-animal-book-by-judith-neumann/1
https://bubblin.io/book/ladakh-by-satie-sharma/1
I've personally read Judith's book on animated ABCs to toddlers in our community. In general, I'd want to optimize for attention span when it comes to kids--Bubblin helps with that. Most importantly, kids are already on the iPad, so it makes sense to go take it where they are.
Thank you for sharing the beeline technique. We can probably provide it as one of the layout template options! :-)