BESW covered the broad concept very well. A couple notes that I think might apply to your specific situation, from my experience participating in, and then running, various religious education classes and camps, which included some roleplaying activities:
KISS: Keep It Short and Simple.
This isn't so much about the ability of kids to understand complex concepts - they can - as it is about classroom management. If you have an elaborate storyline in mind, the odds are pretty good that children, in their boundless energy and creativity, will be off the rails before you can say "What do you do next?" So you probably won't be able to do an RPG Campaign (TM). Instead, think about one question or idea that you want to bring up, and one or two scenes likely to produce that outcome, then call it an episode. You can certainly re-use characters, and should, but don't worry too hard about connecting one session to the next plot-wise. Instead, remember...
Repetition is good.
Have certain verbal cues that you re-use in similar situations. There's a reason shows have theme songs: ideally, it signals "hey remember that fun thing? It's happening again!" I'm not saying you have to sing (though if you do, great), but develop some catch phrases like "What do you think our heroes would do in that situation? Let's use our imaginations," and, yes, "It's time to talk about what we've learned today." Over time, this should help kids get comfortable with the structure so they can be looking forward to what happens next. (Not unique to kids, by the way - I get chills when Wil Wheaton says "I think it's time for you to roll initiative", and my players have agreed that the signal helps.)
Choose your props carefully.
Props are great for building immersion in the activity and helping everybody keep track of who's who (especially if it changes). They're also potentially a distraction. I would avoid anything shaped like a weapon - a helmet can make a soldier, and a blanket can make a shepherd, all without the risk of clobbering or tripping anyone. You also want a curtain or an opaque bin for whatever you're not using at the moment so people aren't pawing through them instead of paying attention. (How strict/careful you have to be about this depends on the extent to which you're accountable to someone who might look in to make sure Serious Learning is happening, not just "messing around". My view is that you can't actually force someone to mentally participate, and it's less stressful for everyone if you don't try, but some folks like to see everybody looking in the same direction.)
Eat the red berries.
You can't lead from behind on something like this - if you look bored and obligated to be there, or even neutral because it's early, the kids will pick up on that fast (I learned that the hard way). Take a few minutes to psyche yourself up before they arrive, if possible. If you make it clear that roleplaying is both fun and okay, they're much more likely to dive in. If you hold back for fear of looking silly, so will they.
Have multiple ways to participate.
Adult players have trouble sitting still and paying attention and patiently waiting their turn and then passionately emoting and improvising dialogue, so make sure your expectations in this area are realistic. Some kids will likely want to stand up and run around the room and talk loudly. Some will be more interested in sitting in the corner where no one is looking at them. If possible, allow for both. Maybe the kid who stutters would love to be the stony-faced guard who has orders not to let the heroes into the castle. Maybe the kid who hates participating in any kind of conflict will be really good at analyzing the emotions and motivations when you discuss the story afterwards (and you definitely should, to bring it home - "What happened when X tried to Y? How did that make Z feel? What could X have done differently?")
Along these lines, I'm going to take a rare opportunity to disagree with BESW, and say that I think even some of the games they listed are too rigidly structured for a mixed group of 2nd-to-6th graders. At least, I think you'd have to take on a more active moderator role to make sure everyone gets to participate in a way that's appropriate for them, and doesn't get lost or drown out everyone else.
Overall, though, this can work and be a lot of fun. Good luck! (And do explore the site and/or ask further questions about running short sessions, roleplaying with kids, etc. as needed.)