"The City of Gold and Lead" by John Christopher. I recall I lucked onto it in the 3rd grade. My gateway book to Science Fiction/Alternate Universe/Fantasy.

The Magic Tree House series kept my kids attention-Mary Pope Osborne

A lot of Jules Verne's work has been edited for younger readers.

As other have stated, let your kids guide you to what interests them. Let's hope they have access to a good reading program in school.

I've found any of the Newbery Award winners from the ALSC are usually good reading. Those librarians are on to something.