If you're playing Touhou or Disgaea games, your idea of a perfect 10 is two five year olds.
Before you suggest that you're there for the gameplay, I'm going to cut you off. You could be playing Ikaruga, DoDonPachi or Radiant Silvergun (for your bullet hell masochism) or the thousands of SRPGs everyone and their mom is making these days. There's plenty of options being provided here, but no, you're pretty fixated on the ones with the little girls and the little boys. This isn't one of those things that leaves much room to believe that it's just a coincidence.
But Alex, these characters are well over the age of consent!Maybe! Fact of the matter is that they still look like a bunch of little kids and that sort of explanation would not exactly hold up if it came down to it. Anyone with more than a couple brain cells would be able to tell you that this is done to not incriminate the creators. There's only see far you can stretch 'You like her for her personality' and I'm pretty sure it doesn't apply to the ones who look like they're 10. You have my sympathy and contempt if you think otherwise.
But Alex, I like the quirky humor in Disgaea!
Put your Invader Zim t-shirt, Jhonen Vasquez hates your guts. Also, Atlus has been doing that for a while now, without pampering to the 'olololo so randum xD' crowd. And even then, there's better stuff out there besides ZaNy CoMeDy antics.
Wait, that's too hard to grasp. I'm sorry.