As I've mentioned before, RPG casts can be some of the most physically varied out there. Your companions can be any species, any race, any gender, any level of intellect, any sexuality, any social class, etc. You can pick up an RPG and have a party of 10 (sort of) average humans, or you could get one with a human, cat fairy, half-harpy thing, floating wise man, wooden guy, flute-playing researcher, flying squirrel, shrimp, odd little robot that looks like a mailbox with arms, and two big genies.* And one of the many factors that vary for party members is age. Oh, sure, at least 98% of the RPGs you'll find are going to have the main character's age range from mid-early teens to very early 20s, but the side characters can be anywhere from 5 to 5000 years of age. You actually see quite a few kids running around in RPG parties--Eiko from Final Fantasy 9, Maria from Xenogears, Mel from Chrono Cross, Roger from Star Ocean 3, and Carol from Wild Arms 5 are just a few examples. And y'know, the more I see it happen, and the more I think about it, the more it seems really odd to me.
I mean, okay, I get that it adds a little diversity to have a drastically younger character in the cast and provides some opportunity for different character development (not that writers often exploit that potential very well). And hey, I have to admit, I've always actually found it kind of cool to have some random, sweet-looking little girl somehow able to kick ass like a crazy viking berserker, even if simple facts of physics and human biology are against it.
But, uh, think about this. We're talking about bringing children repeatedly--the hundreds-of-times kind of repeatedly--into life-threatening combat, where not only do they have to witness brutal acts of violence inflicted upon animals, other human beings, and their friends and possible family members that they travel with, but they have to take PART in this wanton, never-ending slaughter. It's not enough that they're watching Meathead Protagonist #395 butcher whatever fuzzy hell-bunny randomly showed up for a beating at any given time, but they have to murder every enemy they come across like all the other party members. That's actually pretty messed up, or at least it seems that way to me.
Now sure, RPGs only rarely address the psychological issues that come from having to murder human beings for their characters. The fact that a somewhat normal protagonist-kid like Luke from Tales of the Abyss initially has serious qualms with the necessity of taking other human beings' lives may seem like a simple and obvious reaction and opportunity for character development, but it's an issue very rarely explored by anyone in any RPG. So it's not exactly that child party members unrealistically never deal with the mental trauma of battle so much as it is that NOBODY does.
And, to be fair further, it's believable in some cases for RPG kids to be relatively fine with hacking monsters apart with their bare hands, given the kids' personalities. In some cases, a child character, like Anise from Tales of the Abyss or Ken from Shin Megami Tensei: Persona 3 FES, is developed as a character with certain mental processes far beyond his/her age, making him/her mature enough to react to most things as an adult would--in the example of Anise, she's just about the most devious schemer and gung-ho battler in the game, and understands everything with a maturity far beyond her age; she's an interesting mix of a kid's enthusiasm and pluck with an adult's wit and perceptions. In the case of Ken, his readiness for the battlefield is actually worked into his character history. In other cases, the child character is just so daft that they clearly aren't in touch with reality enough to properly consider their actions anyway--Roger from Star Ocean 3 and Choko from Arc the Lad 1 and 2 come to mind. And on the really, really rare occasion, you DO get a kid character who reacts to situations and fighting in a realistic fashion for his/her age--rare as in the only one I can think of is Marona from Phantom Brave, but it CAN happen.
Still, overall, the somewhat common presence of kids in an RPG party just seems odd when I think about it, particularly given how little development they're given regarding their reactions to the constant combat. We don't hear a peep out of Final Fantasy 6's Relm about her having to stab men through the heart with a paintbrush. Does it ever concern Final Fantasy 9's Eiko when she watches Zidane slice up a cute, fluffy (though admittedly diabolical) Yan in front of her? Did anyone in the entire Chrono Cross party ever consider NOT putting the 10-year-old whose hobby is doodling out on the front lines to take on monsters, armed guards, killer robots, and ungodly abominations? It's a weird trend, and, when you think about it, maybe even a bit disturbing.
* I did not randomly make these characters up. I have encountered them all as party members. In the same game, no less. The Magic of Scheherazade was fun times.