I don't understand this "childish" description of FF IX. In fact, despite three of the main characters being kids, it had the most mature and depressing side plot I've ever encountered in a Final Fantasy game.

Their learning of Vivi's nature, who has the personality of a kid, and ensuing desperate search for a "cure" when they realized he was designed to "die" (a plot straight out of Blade Runner) had me caring more about him than any character i'd come across. Anyone who has had a close friend or family member receive a life threatening diagnosis will relate to their journey. Clinging to one hope after another for a cure, being uncertain how to speak to him about it and often falling back on tired platitudes that neither believed, his illness consuming the thoughts of all his friends, and him constantly questioning his nature and place in the world while coming to grips with his mortality etc... If you found this childish you either didn't get far enough into the game or weren't paying attention to the story and dialogue.

To top it all off, they didn't take the easy route out of this one and, just as is often the case in real life, it became clear that there'd be no cure for Vivi. Still, by the end of the game he'd come to terms with his nature and impending death, matured remarkably from the start of the game, and made the most of what time he had left, serving as the closing narrator (having already died). I'm not sure I've encountered a similar story arc (especially involving a kid) that so accurately showed the struggle a person and their friends/family often go through when combatting a potentially lethal illness in any other RPG.

So no, I'd not say this was childish. Convoluted, bizarre "we all had amnesia" storylines like FVIII, or even the "I must avenge the murder of my love interest" trope are more childish. The mere presence of children does not make something childish.