Is there a way to render difficult words into language understandable
at a reading age of 11 or 12? For example (Good News Bible was written
with a vocabulary of 3,000 words -- how did they pick the 3,000?) is
there a dictionary or thesaurus with words only understood by children
that can be used? Even better plugged into MS Word.

I don't think you want a thesaurus, which is for looking up synonyms
and related words.  Sounds like you want a controlled vocabulary up to
a certain grade level.  I remember seeing such a word list for an
eighth-grade reading level once.  I think it was meant for writers of
language arts reading passages.  The constructed passages for that
reading level could use only words from that list.  Is this what
you're looking for?

Archae0pteryx
(not a researcher)

Yes, an eighth-grade word list is exactly what I am looking for in
electronic form, not a book please.