We have several lists on Tales' Treehouse that may help:
Ready for Chapter Books
The lists here contain some of those "skinny books" that are in J but are just one step up from Magic Tree House or Cam Jansen. I checked several of them on the Scholastic Bookwizard, and they ranged from 3rd-4th grade reading level, with a 3rd-5th grade interest level.
Books by Grade Level
Check here for books for 1st, 2nd, and 3rd graders, plus more.
Read the Books
This database has a new interface, so check it out if you haven't played with it in awhile. Remember you can select books by grade level. Each book is tagged with a range of levels (one book may be tagged as 2nd, 3rd, and 4th grade) so a search for a particular grade is more flexible and less restrictive that it seems.
Also, parents always want their kids to be "challenged" but it's good for kids to read a lot on several different levels. If parents are stressed out and not sure if a book is on a good level for their child, remind them that a variety of types and kinds and levels of books is a benefit. Books that are "easy" for children to read give them a chance to practice their fluency skills, build their confidence, and identify themselves as readers...all good things.