Molly Hayes from Runaways, a super-strong mutant 11-year-old girl. Who takes this trope to fairly absurd levels what with her punishing Wolverine and her ability to rip a magically attached cloak through strength. Possibly the Trope Namer, as, though her chosen nickname is Princess Powerful, Chase calls her Bruiser. As for the cute part, well... she's certainly cute. What with those pretty animal beanies she's always wearing and all.
It's been suggested that her powers are psionic in nature (due to the fact that both her parents were psychics, that she glows when she uses her powers, and that she becomes quickly fatigued when she uses them without showing many signs of physical stress) which might explain the magic-cloak-ripping thing at least a little. Tactile telekinesis, in other words.
PS238 has a few of them; the most prominent are Julie "84" Finster, a 7-year-old girl who is a bit neurotic about the fact that she's the 84th F.I.S.S. (Flight, Invulnerability, Super Strength, Super Speed)... basically the superhero equivalent of Brand X. She's also strong enough to wrap a superarmoured attacker in a casino sign and catch punches from a guy who looks to be somewhere in the range of nine feet tall and is identified as having a muscle/mass ratio of "Omega Five" (whatever that means, although given the reactions to this fact, 'bloody strong' seems appropriate).
The PS238 RPG identifies Julie as one of the strongest of the kids at the school, second only to Captain Clarinet when he had his powers. She's basically about as strong as a 7-year-old can be in that world and, as she gains more confidence, the more formidable a combatant (however reluctant) she becomes.
The other is Kristina "Micro Might" Kyle, one of the teachers at PS238 who is able to shrink... and get stronger. Much stronger. To the point where supervillains have a borderline "Oh God. RUN AWAY!" reaction to her.
"The Female" from Garth Ennis's The Boys comic. A petite, vaguely Asian young woman who, when first introduced, takes out a whole house full of mobsters armed to the teeth while alone and unarmed. Her teammate, "The Frenchman", may also qualify, being only slightly larger than her and just as fearsome, although he's male.
Calling the Frenchman "Cute" might be pushing it. He's sweet when he's around the Female, but cute? Not so much. More a case of Crouching Moron, Hidden Badass really. Who would believe the guy who talks to coffee cups, plays games of reverse strip poker and generally acts like a friendly guy is actually an ultra violent and brutal individual who, along with "The Female", is considered the muscle of the team?
In Jeff Smith's "Monster Society of Evil" mini-series, Mary Marvel's physical body remains the same age when she transforms, and definitely qualifies as a Cute Bruiser.
Mary has almost always looked pretty much the same when transformed, right back to the 40s. It's a wonder that she has ever been able to maintain a Secret Identity at all.
The title character of Monica's Gang is a seven-year-old little girl in a red dress, with cute little buckteeth, who is always carrying a blue plush bunny around. Get on her bad side, though, and she will tear you a new one.
In Lullaby, the group's strongest member is probably Red, a little fox-girl who can take on aspects of the wolf. However, at the end of the day, she's still just a little girl, and acts accordingly.
In the later chapters of Bone, Thorn gains super strength among her powers as the Awakened One. She demonstrates this by picking up and throwing a large warrior—easily twice her weight—and by pulling the iron bars of her dungeon cell out of the wall.
Considering that, at least in pre-Crisis continuity, Superboy first donned his costume at the age of eight, he could count as a male version. And he had his powers before that. One "superbaby" story did involve l'il toddler Clark accidentally almost destroying the world.
Cassandra Cain (Batgirl) is a 5'5", 115 pound, pretty, rather quiet Asian girl. She has the remarkable skill of reading body language, but what makes her a cute bruiser is her ability to tank bullets without flinching and punch through inches of quartz.
Hisako Ichiki (Armor) from X-Men is a 5'4", 112 pound, cute and quippy Japanese girl. Her power is Psionic Exo-Armor that can take a hit from anything short of Wolverine's claws, and can lift (with a little assistance) a Skrull Tank.
Hit-Girl from Kick-Ass, a ten-year old vigilante who can kick 7 kinds of ass with a variety of weapons.
Miho from Sin City is cute alright. She's also amazingly dangerous, and will cut you up like a piece of Salami if you happen to piss her off.
Kevin, who Word Of God states is Miho's Evil Counterpart (let that sink in for a moment), doesn't look very dangerous. He's deadly enough that Marv, one of the toughest characters in the series, has to be Crazy-Prepared to beat him.
Laura Kinney (X23) is a pretty, skinny, (officially 5'7" and 110 pounds—100 pounds without her claws—but sometimes depicted even smaller) fanserviceygoth teen whose typical dress style tends to involve lots of low-waisted skinny jeans, tank tops and corsets. She was also raised from birth to be a living weapon, has the same adamantium claws as her "father", and is arguably one of Marvel's most lethal killers. Marvel: Avengers Alliance even has her in the Bruisers class. This is the same category as Hulk, the Juggernaut, and Thor.
Molly Danger, the perpetually 10-year-old, super strong protector of Coopersville. Very cute, very capable bruiser.
Violet of the Rat Queens is a slender dwarven woman who carries an almost absurdly-large melee arsenal and is highly skilled with its use. She also shaves her beard, presumably to have a better chance at getting a date in a surface world that assumes that its women come without facial hair.
Li San O'Rourke of the first Atari Force series is the team's Tactical and Security Officer who pretty much knows how to defend herself. She eventually teaches her daughter Erin Bia (Dart) how to be a capable fighter in her own right.