This series reading order originally started out with the “extra long” books (some over 600 pages), but they don’t appear to be available anymore. The author has re-edited and condensed them saying:
“Baby Series ‘Lite’ is the anniversary edition of last fall’s Baby Series. These Lite editions have been condensed from the originals to accommodate those readers who prefer a shorter read. The essence of the story is the same, but its length has been edited and bonus chapters added on Book 2 and 3!“