Since you've put this under 'writing for kids', what got me as a kid was any mystery, the supernatural, magic, or promise of far-off adventure. Characters that pulled pranks or had some kind of anti-authoritarian streak would get me reading as well.
Because I lived as a child in places with libraries that had plenty of older books and few more modern one I'm less familiar with 'newer' MG or kid's books.
I probably read nearly every Pippi Longstocking or Mrs. Piggle Wiggle book, most of the Narnia and Wizard of Oz series, Mary Stewart, and about half to three-quarters of what Enid Blyton ever wrote (including the non-magicky Mallory Towers).
My mother tried to get me to read things like Anne of Green Gables, Little Women or Judy Blume and I could never get into any of them. I also couldn't stand characters who were too 'goody-goody'.