You ever see "It's A Good Life"? It's one of the bleakest episodes of the Twilight Zone, and Joe Dante even did a version of it for Twilight Zone: The Movie. Much as I love Dante, though, it's best to skip his remake and stick with the original, a nasty story about what happens when a little boy has all the power in the world. The boy isn't evil, exactly. Kids usually aren't. Evil requires an understanding of consequence and mortality, the knowledge that choices have lasting, and often impossible to predict effects, and kids don't start struggling with that sort of thing till puberty. But the little boy in "Good Life" might as well be evil, given what he does to his family and the people at hand. Because no one is able to discipline him, he acts out constantly, and his slightest whim has a horrible, hateful reality. Living around a kid like that, you'd spend the rest of your (probably short) life trying to appease a will and a mind that, given its lack of maturity, won't ever stay appeased for long. And once you make a mistake, that cornfield, from whose bourn no traveler returns, is only a wish away.
Jesse, the cause of Sam and Dean's woes in "I Believe The Children Are Our Future" (and a curse on the Supernatural folks for getting that song stuck in my head), isn't quite as obnoxious as Bill Mumy was in "Good Life," but his beliefs have the same devastating effect on the locals. He thinks itching powder makes you itch obsessively, so a young woman scratches her way right into her brain. A guy gets electrocuted on the world's first non-Joker related joy buzzer death, a guy gets his teeth yanked out by a really butch Tooth Fairy, and somebody holds a face that actually sticks like that. Oh, and Dean gets hairy palms; draw your own conclusions.
The first part of "Children" didn't really grab me. We've seen this kind of thing before, starting with the first Loki episode that had urban legends coming to life. Sam even mentions tricksters while he and Dean are trying to figure out what's causing the problems. Things get a lot more interesting, though, when they determine the source of the problems, Jesse, an eleven year-old kid who's a lot sharper than you'd expect. It's funny, really—he immediately questions Sam and Dean's FBI identities (Page and Plant? Man, Dean isn't even trying this week), but he buys his dad's explanations, like the killer joy buzzer, without question. Because that's what kids do, at least the ones that come from good parents. When they start getting paranoid, it's outsider information they question, because asking the hard questions about one's parents means risking what keeps them safe.
Jesse has a good reason to want to be safe, too. He's a "special" kid; his birth mother, Julia, was possessed for the nine months she bore him, a virgin birth that resulted in a half-human, half-demon child. (It also helped her get free of the demon, as the pain let Julia briefly get control of her body long enough to eat a bunch of salt she had lying around.) Cass pops in to explain that this means the kid is an anti-Christ. Not the actual son of Satan ("Your Bible gets more wrong than it does right."), but still a potent enough threat to the host of Heaven that Cass thinks he needs killing. He fails at this, as Jesse turns him into an angel action figure (if I was Misha Collins, I would so steal that prop), and Sam and Dean arrive too late to stop it. Then Julia shows up, re-possessed by the same demon as before. She wants to recruit Jesse for her side.
"Children" doesn't get too deep into major themes, but it does spend some time on the lies that parents tell their kids (by the end Dean finds himself wishing their father had told them a few more), and how sometimes the truth is the only way, no matter how difficult it is to tell. Sam saves the day by giving things to Jesse straight, and here's where the episode stops being like the original "Good Life" and starts being more like Dante's version. Only in a good way; Supernatural couldn't really deal with the existential bleakness of that original story (odd thing to say about a show where one of the main characters has spent extensive time in Hell, but there you go), so clearly Jesse had to be won over or defeated in some way. To win him over by being honest with him was a smart choice, and it was especially smart coming from Sam. That's always been his one advantage over Dean—Sam's always been more comfortable with the truth than with a lie. That can be tedious at times, but here it worked just fine.
There are a few little things that didn't quite work here. The first act was too easy in the way Jesse's powers were manifested, and I would've liked to have seen more of Jesse's adopted parents than the brief glimpse we got at the end. And the solution, with Jesse wishing himself away to, I'm guessing, Australia, was on the convenient side, although the melancholy aspect of a child having to protect the adults who were supposed to protect him gave it some weight. But this was generally solid, and an interesting expansion of the Supernatural universe. Wonder if we'll be seeing the world's friendliest anti-Christ again before the end of the season?
-"Children" also owes a debt to Good Omens, by Neil Gaiman and Terry Pratchett. If you haven't read it, you should; it's a humanist, gently comic take on Armageddon, with another child anti-Christ who isn't quite as evil as he's meant to be.