I don't think it would be, as long as it's handled in an age-appropriate way. This doesn't mean all parents or parents' groups will approve, or that all agents or editors will either, but I don't think there's any industry-wide rule against it, unless you are writing specifically for a religious audience. As for zealots trying to get a book banned, I wouldn't allow that to stop me from writing anything. For one thing, it would allow the zealots to win. For another, a little notoriety sure as heck didn't hurt Harry Potter or other books that get on banned lists. A book has to have a certain amount of popularity before it can attract the ire of these groups, of course, but once it does, it might even be good for sales. One reason I read HP is because I figured anything that crowd hates can't be all bad.
I've run across references to the Devil, or even Satan, in MG books. I remember in one of the Great Brain books, the characters played a prank on another kid where someone dressed up in a devil costume and pretended to be there for his soul (because the kid backed out on a childish, make-believe oath he took on the Bible). While it was portrayed as a dumb thing kids do (since the oath on the Bible wasn't technically binding in that situation), there was no doubt in any of the characters' minds that the Devil really existed and your soul would be damned if you lied under a real, binding oath given by a priest or justice of the peace.
Kids often believe in things like devils and demons very literally, even if they weren't raised to be religious. I still remember the stories my cousin told me about the little mica flecks in some sidewalks being "witches candles" that had a small chance of pulling your soul down to Hell (though I was younger than MG then, she was about that age). It can be fun to be scared.
I'm pretty sure I read books with references to selling one's soul when I was a kid.
There are plenty of stories for MG kids that deal with paranormal or fantasy too, where things that are dark or scary are objectively real within the story universe and are real threats to the characters.
Of course, you'd have to handle it in an age-appropriate way, with less gore and graphic-ness being appropriate than for YA or older. But there are MG novels that are pretty scary in their treatment of the supernatural, with lots of tension and potentially dark consequences. Think of Neil Gaiman's Coreline. My nieces loved it when they were 8-9.