South Italy, also known as Italy Romano, was spoiled, lazy and entitled as a child, with a habit of insulting his caretaker Spain and rejecting all of the kind acts he attempted to do for him...and then grew into a very fierce mix of allTsundere Type A varieties.
Sealand transforms from a somewhat delusional and determined Cheerful Child into this when he's around his older brother England, whom he refers to as "jerk England," and goes out of his way to tell him that one day he's going to be a bigger empire than England ever was.
More recently, Ladonia is shaping up to be a prime example, mixed with him being a Spoiled Brat prone to throwing tantrums when things don't go his way. He demands to be made leader of the micronations in his introduction, no dissent tolerated.
Czech Republic was this as a child; she was Always Someone Better to the young Slovakia, and enjoyed rubbing it into his face and teasing him.
Break Blade: not necesssarily a kid, but Girge deliberately admits this.
Rygart: "You're basically just a bratty-kid!"
Girge: "That, I already knew."
Meilin from Cardcaptor Sakura. When she isn't fawning over Syaoran, she's constantly bragging about her abilities, real or imagined, and early on, whining whenever Sakura gets a card over Syaoran and just being a general nuisance. However, she is also very determined, caring, and always ready to take action, traits which become more prominent late into her appearance in the series.
Mashiro from the yonkoma (and its anime adaptation) Engaged to the Unidentified. Self-important, and always trying to find a way to either take advantage of, or inconvenience her prospective sister-in-law Kobeni. Thankfully her bratty instances are curbed either by her own childish tendencies, or by Kobeni's older sister Benio, whose attentions Mashiro is terrified of.
Mai and Mami from Explorer Woman Ray, a couple of treasure-seeking teenage girls who steal an artifact from the title character left back in Japan, in the hope of meeting her and returning it.
Girls Bravo: Tomoka. She is portrayed as super-irritating. In fact the only justification for her terribly annoying attitude and disposition is that she is a little girl.
The main character of Hakkenden Touhou Hakken Ibun, Inuzuka Shino, is this. Of course, he's Older Than He Looks due to possessing Murasame as the price for being his vessel and wielding his power is to have one's time frozen. Aka, he's actually 18 instead of 13. Him being a brat is semi-justified by Who Wants to Live Forever? and the issues that come from always being seen by others as a kid. And, unlike most Bratty Half Pints, he is actually probably one of the strongest characters.
Shippo in InuYasha zigzags between this and Mouthy Kid. In his defense, there are times when he tries to help (even though he is too young) and his teasing of Inuyasha is often due to him upsetting Kagome (who Shippo sees as a adoptive mother).
Naruto's prototype was even brattier and far more of a Jerkass. In order to take revenge on a boy who spread rumors about him, he trapped him in a hole, pretended to offer him a lift out, then urinated on him, leaving a stench that lasts 10 days.
Konohamaru, the Third Hokage's grandson, is a more straightforward example. Although later on, we see him Take A Level In Badass and give a Pain body a Rasengan.
Yuuri of Mermaid Melody Pichi Pichi Pitch is a brat who continually insists that she isn't a child and stomps off to go do her own thing, always failing miserably. After she leaves the cast (for the most part), Seira is introduced on the good side; she does mature, and was only selfish because she had yet to gather the facets of love (which was the original intent of the quest for her to be born; having her heart stolen complicated things).
Mannen in Prétear, the self-proclaimed leader of the younger half of the Leafe Knights. He tends to get the group in trouble by demanding to be treated as equals with the older knights, but he seems to genuinely mean well.
Also, Akaya of Rikkai. It's developed so far that Brat is basically his second name in fanfiction.
Anita King of R.O.D the TV, at least partially (especially prominent in the anime's early episodes).
Yahiko from Rurouni Kenshin is an interesting case. He's obviously the Bratty Half Pint, and is not only fully aware of it, but doesn't want to be. His character arc is his journey from Bratty Half Pint to full-fledged Shonen hero.
Chibiusa of Sailor Moon, at her worst. More in the anime, but the manga version wasn't above everything.
In Saint Beast, Maya is the youngest and most immature character on the show. He tends to throw tantrums when things don't go his way, and is often a Distressed Dude.
Komachi from Samurai 7; she does get along with most of the others, but dislikes Kyuzo. But then, no one gets along with him. It is notable that Komachi almost literally hangs off Kikuchiyo.
Sgt. Frog: Karara, Chiroro, Tororo, Taruru (to some extent) and Sumomo (in the manga).
Through flashbacks and Kululu's age manipulation beam gun, we learn that Fuyuki used to be the brattiest of them all.
Miu from Strawberry Marshmallow. Rare is the episode where she doesn't earn Nobue's wrath, usually in the form of being knocked face-first into the floor.
Uriah from the second season of Superbook was this way to Chris at first.
In the first OVA series, Sasami of Tenchi Muyo! was this, her first appearance being spent playing a prank on Tenchi and insulting Ryoko. This held over into the first Pretty Sammy OVA, but once the second Tenchi Muyo! OVA series came out, she came out of this and became the more mature of the girls. The whole Tsunami thing probably helped with that.
In Umi Monogatari, Urin is childish and petulant at times, moreso to people on land than her mermaid sister Marin.
Ten from Urusei Yatsura. One translation of his full name, Jariten, literally means "Bratty Ten". His insistence on "Ten" is for much the same reason Sakuramboo, the ugly little Buddhist priest, usually calls himself "Cherry" — Sakuramboo can mean "Cherry" (as in the fruit — Sakura refers to Cherry Blossoms), but also "Deranged Priest".
Johji of Yu-Gi-Oh! — He is bratty and, of Yugi's party, the only one who he gets along with is Anzu... because he is a perverted baby who lusts after women.
The Nishigori triplets from Yuri!!! on Ice, though most of their bratty behaviour is aimed at their mother Yuko. They use their parents' social media accounts without permission (like how they used Yuko's account to upload the viral video of Yuri reenacting Victor Nikiforov's program, which she scolds them for afterwards), and they often stay up past their bedtime to watch figure skating competitions and post on social media, much to Yuko's frustration.