Maybe that stats for the devil are ruled out, but he still is a character of Folklore. Even related to Jack O lantern. What about his minions: the seductive Succubus and the little devils that are often pictured in paintings. There are also a lot of other creatures named in the book, like werewolves. Maybe the denizens of the underworld are a possibillity: zombies, mummies and perhaps something even darker. Vampires are also often used in folktale but, like the werewolf, a bit cliché. Stats for more fantasy like creatures, such as a lesser dragon to guard a prinsess, can be useful as well.
I like the 'cat' idea for the mirror monster, it makes it much more of a threat indeed. I'm going to include that for my next story.
For the Genie, they sometimes make a difference between an Efreet and a Djinn. The Djinn being the more kind (but often still tricky) of the two.
Or maybe the concept of an 'evil' genie just developed because of the confusion with genies attacking and then apologising.
I just did the cat thing for a bad pun, to be honest.
While I think we should introduce stats for the Devil and most of his servants, I'm hesitant about the succubus. It could get a bit gratuitous, and the children don't even have serious hormones yet. Just an idea: Maybe a succubus isn't actually beautiful, but possesses some quality that makes her seem such to the residents of Grimm? She would therefore have a lot of minions and could be a nasty enemy.
An idea for the little devils: They tempt mortals from the Real World with magic, for 'no price'except the risk of estrangement. If a child succumbs to full estrangement and then dies, he becomes another imp in the Devil's service.
There aren't many dragons nowadays, but they should have some stats. They'd basically be powered-down versions of the Dragon.
Werewolves should be the results of humans who allied themselves with the Big, Bad Wolf. Slowly, they devolve into wolflike creatures, until it's hard to tell which he is. At this point, their descent into madness is complete (maybe the Moon takes their souls?), and they are quickly hunted down by the real wolves, since they are now a threat to the Pack. Sometimes, though, they escape into the Checkerboard Kingdoms...or find some yummy children to bring back to the Pack, and thus prove their worth.
I like the idea of adding a few monsters to the Underworld. Zombies should be back to their old selves: Unkillable, so all you can hope for is to get awayor them into the Styx. For some reason, the waters purge the abominations of their taint, releasing their soul. Of course, they know this, and aren't going near the place.
Mummies used to nobly guard their resting places, but times change. Now, thieves have the sense to stay away, and they just don't see the point (being rather smarter than zombies). They've taken up guarding the Rotten King's palace, they see quite a bit of excitement there. They might even be willing to help unlucky prisoners. If they can be shown an alternative guarding station, that is.
I don't know about vampires. Cliches are fine, but they've been made more 'original' so many times, it's hard to get the same element of 'twisted'.