A German childrens' detective series, and lately, a series of video-games as well. It is one of the most popular kid detective series in Germany, along with The Three Investigators. The title refers to the initials of the four main characters, Tarzan/Tim, Karl, Klößchen, and Gabi.
TKKG provides examples of:
Adults Are Useless: Usually played straight, averted with some of the more important adults, for example Gabi's dad Kommissar Glockner.
Amateur Sleuth: The titular group. Partially justified that some events are not of a scale where Police would get involved. Other times they support the Police with investigations of their own because friends are involved.
Ax-Crazy: Alternate Character Interpretation for Tim, especially if you look at his track record and willingness to use violence. (But take this with a grain of salt: The kids have solved hundreds of crimes so far and barely aged a year, and also have no realistic Character Development, so how probable is this after all?)
Damsel in Distress: At least a third of the stories put Gabi into some kind of danger from which she has to be saved by Tim.
Do Not Call Me Paul: After he couldn't be called Tarzan anymore for legal reasons, Tim stated that he had seen the Tarzan movie, didn't like the protagonist and didn't want to be called by his nickname. In the German version, he also doesn't like his given first name Peter, so he prefers being called Tim, which is short for his second name Timotheus (after his grandpa).
Double Standard: Gabi is usually sent home if it gets late or dangerous. The boys can do what they want.
Good Bad Translation: Given the translation largely being for the benefit of Germans, catchphrases abound, including - "Is it true that you have the best machines of anybody in the town?". Also, rather archaic slang, such as "They've given her the Mickey Finn".
Identical Stranger: This trope happens in TKKG stories more often than statistically probable.
In-Series Nickname: "Tarzan", "Computer", "Klößchen" and "Pfote". It's never mentioned who actually invented them.
Kid Detective: Kids solving mysteries at least some problems concerning authorities are averted because they have contacts who know they are trustworthy.
Kung-Fu Kid: Tim qualifies, being just 13 / 14 (forever). Strictly speaking, he doesn't know Kung Fu but Judo and Jiu-Jitsu instead, but he's still pretty good at them for his age.
Long Runner: Started in 1979, more than 167 books have been published by now. There also more than 100 audio books.
Whodunnit: The second book. A classmate of the gang his disappeared, and there are several suspects: A blind clairvoyant who fakes his blindness, an Italian restaurant owner, and a bully from their school. All of them are pretty unsympathetic and have possible motives. At the end, they find out that the victim "kidnapped" himself, because he was fed up with his parents; his father is preoccupied with his work as an architect and his mother spends her time imagining she was a reincarnated medieval noblewoman.