Modern kids are busy, and may not spend the time pouring over rulebooks that older gamers remember doing at the same age.
Some tropes hold true, even when you aren't exposed direclty to gaming-genre. These kids had no exposure to D&D and the associated fiction. But they still chose the half-elf, last of her tribe, spent years in the wilds alone before returning to civilization dragon-blooded fire sorceress, and the drow rougue/assassin with purple eyes.
Something I already knew, but got to see first hand - young women can take vicious glee in combat.
Something else I already knew - if you give the goblin a cute name, and have it talk in a funny voice, the viciousness goes away.
I help run a church roleplaying group (we call it a D&D group, but I switched to Big Eyes Small Mouth for my system) and I took on the younger table; my players are aged 9 through 14 or so -- I have a pool of about eight and usually get 3-5 on any given evening. It is both rewarding and illuminating running for them. Some memorable moments:
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GM(Me): "You see an orc charging towards you"
Player: "What's an orc?"
Echoing Umbran -- you cannot assume that the players have exposure to anything you have. Some of them haven't seen any Harry Potter films so one player's explanation of an orc ("It's like a really big and angry Dobby") didn't help. It is surprising to find out how many things you thought were common knowledge just plain are not so.
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Umbran's comment on the cuteness reminded me of how our campaign took a left turn. We were running Living Forgotten Realms mods for a while and we mentioned that the next adventure took place in the Dragon Coast. Here's how things went:
Player: "So there are Dragons there, right?"
GM: "actually, not particularly"
Other Player: "If it's called the Dragon Coast, there must be dragons there"
GM [remembering 'yes, but']: "Yes there are, but they are pretty rare"
Yet Another Player: "We should find them and get a baby dragon"
All other players: "YES!"
GM [bowing to the crowd]: "So the merchant who needs his caravan guarded is willing to offer ... ummm ... information on baby dragons as a reward"
All Players: "We will do it"
This ended the informal "let's play LFR mods" part of our campaign, and the YEAR-LONG quest to raise a baby dragon started. Interestingly, the elder kids were totally on board with this also, and ended up as dragon-riders
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A surprise for me was how much the kids got into debate sessions. They spent a solid two hours seriously engaged debating how to frame a constitution for a confederation of towns. My favorite quote from that session (from a 12 year old): "I want to get back to the discussion of whether to allow each town to negotiate individual trade contracts or require that they be agreed by everyone".
Don't assume that kids will be bored by politics ...
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Romance, sexuality and gender issues. This will be a thing. Be ready to handle players being experimental in-game as a way of exploring things. Not going into details here, but if you describe a person as "cute" or "attractive" I have found that it's quite likely that someone will decide that the goal of today's game has shifted for them. And the difference between 9 and 14 year olds is immense.
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It's been very interesting. My own kids are 19+ now, so not so much kids. And they grew up knowing orcs from goblins, and able to add numbers up to 20 from a very early age. So it's been very educational for me, teaching me not to stick to preconceived notions and genre tropes. My final quote is from one young woman who went first in initiative in a 4E combat encounter. I told her the horde of bandits were charging her team and asked what was she going to do. She replied:
"I'll hire them"