Corpse Party has Yoshikazu Yanagihori, former Cool Teacher. As his mental illness progressed, he reverted to a childlike mental state, and eventually couldn't even convey his thoughts. People noted that he was still the same person he was before, so it was shocking when he apparently abducted four children and brutally killed three of them. He didn't actually kill the children. He was manipulated into kidnapping them, but he was just an accomplice to the real killer... the surviving child, who was actually a malevolent spirit.
Yellow Heart, from Hyperdimension Neptunia Victory. Despite being physically powerful and having the visible age of the other CPUs (at least), she does everything to please her "parents", can only read and write very simple words, and is generally sharp as a sack of particularly dull hammers. Turns out she's an artificially altered Peashy, and therefore actually a child, despite "the big me" looking like an adult.
Tingle from The Legend of Zelda series. A 35 year old man who believes he is the reincarnation of a fairy and spends his spare time floating around on a balloon searching for other fairies. Ramped up in The Legend of Zelda: The Wind Waker, where his HQ is a tower where his cohorts spin the top around to make magic happen... or something.
Kyousuke from Little Busters! is a benevolent, more subtle example: because he's so obsessed with finding fun things to do, tends to be overdramatic and switch between extreme emotions, and doesn't always possess the greatest amounts of common sense (though that's not unusual in this game) he's commonly compared with a child.
While on the subject, Mario himself easily fits into this category. Aside from being a never-ending fountain of bubbly joy, He has absolutely zero qualms with playing with the Mini Marios his own company manufactures, indulges himself in any kind of fun activity regardless of how juvenile, and seems perpetually impossible to take the life-threatening obstacles he overcomes with anything but a whoop of excitement and charging headlong at it.
Wario, as if it wan't obvious enough.
Justified in the case of Grunt from Mass Effect. Since he didn't technically HAVE a childhood (unless you count accelerated growth in a vat while a Mad Scientist plays context-free videos for you to be a "childhood"), he comes out of the tank nearly fully grown but without a great deal of comprehension of the world beyond "if you squeeze this thing here, the gun goes bang and kills something". As a result, while he's fairly smart, he still plays with action figures, pulls out Buffy Speak on a regular basis, and mostly views Shepard, the player character, as a Parental Substitute who will teach him how to do all the important krogan things like break stuff and set it on fire. In the Citadel DLC, after he sets a Citadel Security car on fire, steals it and crashes it, this exchange can happen:
Shepard: Grunt, apologize to the nice man for setting his car on fire. Grunt: (looking at his feet) I'm sorry I set your car on fire... [notices Shepard's expression] Grunt: ...and I won't do it again. [...and as soon as the police have left] Shepard: I love you, Grunt.
Big Boss, of all people, in Metal Gear Solid 3 and Metal Gear Solid: Peace Walker. He has a Phobia of vampires, he gets ridiculously excited about eating things, he makes childish noises of disappointment when told not to eat things, is rather fixated on his mother figure, thinks making cardboard tanks and using bananas as firearms is a pretty neat idea, gets along as equals with a literal child, and still believes in Father Christmas.
Heck, N's upbringing in general does a lot to explain his Man Child tendencies, especially when you see his toy room.
Doctor Ivo "Eggman" Robotnik has shown some shades of this throughout the Sonic the Hedgehog games. In spite of his advanced knowledge of science and robotics, Eggman also appears to be very immature and is prone to throwing childish outbursts whenever something doesn't go his way. His television and comic depictions also exhibit some of these traits, most notably his Adventures of Sonic the Hedgehog counterpart.
Luke fon Fabre from Tales of the Abyss starts out very gullible, emotional, demanding, self-centered, petulant, frustrating and stubborn. Most of the time, he acts like he's seven years old. Which makes sense, because technically, he is. He was cloned from the real Luke fon Fabre seven years prior to the game's opening, and retained none of the original Luke's memories.
Max Hass from Wolfenstein: The New Order is an almost literal version, due (presumably) to the fact that a good 1/4 of his skull is missing. His room is filled with toys (some of which you can go out of your way to recover for him at the base), and he's got an acute fear of loud noises (namely buzz saws).