Lots of good suggestions here-- so many so I am blanking on anything to add. Except to say that opening up the time period, and instead allowing your kids to play ANY child hero, gives you a whole lot more creative room.
With regards to the subject matter of Slavelords, I think if you portray the sli'ess as monsters/slavemasters more in the "Cinderella" mode, you'll be ok. There are some other icky parts of the adventure that should/could be glossed over or hand-waived. It is not so much that Fairy Tales do not have adult situations, but simply the manner of presentation. How may children's stories involve monsters willing to torture, kill, and
eat small children, after all? As an adult, I'd turn up my nose at an adventure that put children in danger-- it's just not the sort of thing I care to play at. But as a child? Could I get into that? Of course-- most of my youth was spent reading stories of just such peril.
Honestly, I don't think it's as much any particularly adult subject matter in the book that will be troublesome, but rather the VERY political nature of the whole thing. I don't know your kids, of course, but I think some parts of the adventure will require them to think in ways beyond their years (or ability). Some adult authority figure NPC might be important to help them along.
I don't own Grimm and have never seen it, but I do believe it's d20 based and it might be a ruleset you could use in lieu of Grim Tales; depending on exactly how much it's d20 based, it should still work with the adventure laid out in Slavelords.
I have to say I'm flattered that you are enjoying your read of Slavelords enough to shanghai your kids into playing through it.
