The pain staking selection of every single word, syllable and its age appropriation is a time consuming part of writing children's book. Its not to be rushed however. Every word matters. Each sound, syllable, sentence as important as the next. This process is especially challenging with picture books where word limits are only 600 words for the entire book. That's a mere 20 word per page average for a 32 page spread (excluding the cover pages). Each word so carefully selected.
Wouldn't it be great if someone produced an app that suggests words suitable for different reading levels or lists of synonyms? I haven't found an app yet but I did discover this excellent book called simply Children's Writer's Word Book, 2nd edition written by Alijandra and Tayopa Mogilner. While writing or editing your picture book or early reader draft, you can search for words that relate to your target audience's reading level. The book has synonyms for words at each level and are presented by a child's grade ie Kinder, Grade 1, 2 etc. It includes guidelines on sentence length, structure and word usage for each level as well as how to (or not to!) tackle sensitive topics of diversity, racism, religion and appropriate work usage. There is a section on the US National Reading Standards which is still useful here in Australia.
I bought my copy off Amazon in January 2017 for less than $30 (incl shipping) and its dog eared already. As much as an app would be more mobile, I discover more flicking through the pages and I can sticky note the pages I refer to most.
Highly recommended resource for new picture book writers.