Children are prepubescent humans that wander the streets with adults. They were supposed to appear in every Grand Theft Auto game except the "top down" ones (1, 2, Advance and Chinatown Wars). There are references in all the games that support their existence, the two with the most evidence are Grand Theft Auto IV and Grand Theft Auto V.
Grand Theft Auto III
It has been confirmed that the beta version of GTA III had children as well as drivable school buses, but they were removed in the final version, due to controversies that may arise from having children in a violent video game.
Various pre-release screenshots of GTA III show school buses driving on the streets of Liberty City. The school buses came complete with light-adorned rears and a bright yellow paint job. However, these school buses were omitted from the final version of the game. No children were ever physically seen in any pre-release screenshots or artwork for the game, and there is no trace of any children NPC models in the game's files.
In the final version of GTA III, there are several clues that support the existence of children in the beta version. There are wrecks of school buses around 8-Ball's compound, and near the Harwood Car Crusher. The same exact bus wreck model continued to appear in later games, including GTA Vice City (inside the Hyman Memorial Stadium during "Dirtring", and in Little Haiti and Little Havana), GTA San Andreas (in Blackfield Stadium during "Kickstart"), and GTA Liberty City Stories (The same locations as in GTA III).
Grand Theft Auto: Vice City
One of the clues regarding children in Grand Theft Auto Vice City (besides the school buses) is a sign on a wall on Prawn Island that says "Caution children at play". The sign is located in the Abandoned Park. Another sign reading NO KIDS-DOGS AFTER 7 PM, can be spotted near the bar counter at the Greasy Chopper. Posters of an unmentioned movie or TV show at the Greasy Chopper, named GO BANG!, features an anonymous child artist. Maude Hanson formerly worked at a children home, as mentioned at Rockstar's official website. Children are also cautioned to swim alone in a pool at Vice Point.
Grand Theft Auto: San Andreas
In GTA San Andreas, if you drive near a certain pedestrian at high speed, you can hear her scream "MOMMY!". This might either mean that Rockstar wanted to include children in GTA San Andreas, or just set up the civilian to react that way, possibly as an Easter egg. In Ammu-Nations there is a sign board about children.
Inside Old Reece's barber in Ganton, Los Santos, there is a photo, which consists of three people. A tall person who looks like a male, a medium high person who looks like a female and the shortest, who looks like a girl child. The photo is located on the window in the barber store. It can be easily seen when the player is exiting the barber store.
Grand Theft Auto: Vice City Stories
The only appearance of minors is witnessed in Grand Theft Auto Vice City Stories. Mary-Beth Williams is one of two minors to appear in the series, the other being Jill Von Crastenburg. Mary-Beth is the newborn baby of Louise Cassidy-Williams and Marty Jay Williams. She appears several times in VCS, often in her mother's arms.
Grand Theft Auto IV
There are various abandoned baby strollers sitting around trash piles and beaches in the city, hinting that not only children are in the game, but babies as well. One of these strollers, nicknamed the Haunted Stroller, is the subject of a particular myth in the game.
School buses reappear in GTA IV as non-drivable wrecks. These school bus wrecks appear in two locations in GTA IV. In the junkyard in Northwood, and on Firefly Island, close to the Screamer roller coaster. Also, there are traffic signs around the city directing traffic to stop for school buses.
Also, the Carnival on Firefly Island could indicate the presence of children. There are various rides in the carnival which appear too small for a full-grown adult to use.
There is little proof besides these clues that support the existence of young children in Grand Theft Auto. However, many players and myth hunters still believe that children can be found in-game. The only clue that is yet to be found is traces of a child pedestrian model in the game's internal files.
While young children do not physically appear in-game, it was confirmed that they do exist. When watching TV, ads for Bullworth Academy from the game, Bully (a game by Rockstar Vancouver), appear occasionally. It was confirmed that Bully takes place in the same universe as GTA. There is also a child pornography sting site on the in-game version of the internet, "littlelaceysurprisepageant.com". The site does not have any actual pictures of children (inappropriate or otherwise), but rather has the logo of the Liberty City Police Department and the text "This site about children's beauty pageants has been shut down. Your IP address has been logged, and we know who you are", against a plain white background. Visiting the site results in an instant 5-star wanted level. The site is mentioned on radio news broadcasts.
There are, however, teenage girls that spawn frequently in Rotterdam Hill, among other places. They are known as "SOCIALITE" in the game's internal files, and they look a lot like Jill Von Crastenburg, who appears on the in game TV show I'm Rich, and is said to be 13 years old. They also have a young sounding voice. Viewing the texture files in Spark IV, or (if the one is a pervert) by looking up their skirts in game, one can see that, unlike most of the pedestrian models wearing open skirts, the young ladies are wearing underwear, likely due to the controversy that would ensure otherwise. This also means that the characters seen in game are likely not Jill herself (since Jill is specifically said to not wear underwear on in-game celebrity shows), but rather middle class teenaged girls, who wish to look like Jill. Also supporting this theory is the fact that, unlike the in game pedestrians, Jill is often followed by paparazzi, and the fact that more than one of them often spawn at the same time. Additionally, there is a pedestrian model known as "SHOPPER" in the internal game files, who doesn't look quite as young as SOCIALITE, but screams pleas such as "Daddy help me!" and "Oh my God, I've been shot, MOMMY!" when threatened. SHOPPER may be somewhere from the mid teens to early 20s.
Grand Theft Auto V
In a pre-release screenshot of GTA V, an elementary school was shown, called the Los Santos Elementary. This led players to believe that children were going to be featured in GTA V. However, children do not appear in the final version of GTA V (with the exception of Michael's children). The school does. Like in past GTA games, children are only featured in radio shows, advertisements and in GTA V's case the Internet.
On the website "Accept-The-Truth.com" if you click on the "Play God" link, it will take you to a slideshow. This slide show includes a family of 4 people and a dog. Two members of this family look to be children, one looks to be a teenage boy and the other is a little girl.
Michael de Santa's house contains photographs of his family through the years, including childhood images of Tracey and Jimmy. Franklin Clinton can be seen as a boy in pictures at the Clinton residence.