Once upon a time, way back in the crazy days when I was into 3rd Edition D&D and frequented the Wizards.com message boards, they had a section for essays, research, and more formal stuff that required authorization for all posts by the moderators. It was called Redgar's Repository. I have no idea if it survived the 3E to 4E purge, er transition, or whatever it was, as I'd long since stopped visiting that little chunk of the internet.
But my little contribution to Regdar's Repository was an article listing the sources of the monsters in the 3E Monster Manual. As best I could, I searched my books of mythology and folklore, online sources, and asked for a lot of help from other people in the know.
Of course, there was (as one might expect) debate on the origins of some of the monsters. Since monsters have changed over time, some names get recycled from time to time, and two differnt D&D monsters are sometimes the same monster in mythology, it can be hard to get a definitive answer for some creatures. Add to the fact that some monsters were created whole cloth, and others are basically created by some TSR writer but given the name of an actual monster, and there's bound to be confusion.
One of the funniest complaints I had in the thread about my article was about the Cloud Giant. It's obvious that Gygax or Arneson modeled it after the giant in Jack and the Beanstalk. Lives in clouds, smells really well. One poster just refused to admit it, and tried arguing left right and center about some other origin, how the mighty and regal Cloud Giant could never be based on a kiddie fairy tale! Of course, I challenged him to show me another source of the monster, and he couldn't.
Well, I was reminded of that because we're reading Jack and the Beanstalk in one of my kindergarten classes. Another class is reading Hansel and Gretel, another Puss-in-Boots. And it brought home to me how much monster slaying/treasure looting there is in old fairy tales, in addition to reminding me of that old thread on Regdar's Repository.
Anyway, if you've got kids too young yet to dive into Robert E. Howard or Fritz Leiber but want to get them interested in D&D, a good Fairy Tale inspired campaign might work wonders. And you may be surprised at how well kids brought into the game that way take to the idea of outsmarting the monsters rather than just charging in with swords drawn!