The Immigrant Kids are minors who have been forcibly separated from their parents and detained due to the immigration policy of the United States. Most of them are Hispanics who snuck into the United States from the Mexican border with their parents. They first appear in the Season Twenty-Three episode, "Mexican Joker".
Due to the change of immigration policy in the United States, the Immigration and Customs Enforcement (ICE) arrest a large number of immigrants suspected of entering the United States illegally. The minors are separated from their parents and detained at a detention center in Texas. As anyone can give an anonymous tip-off, Cartman gets Kyle's family detained and Kyle gets sent to the detention center. Later, Stan gets Cartman sent there in revenge.
The living conditions at the detention centers are very poor. The immigrant kids sleep on the ground and are only given aluminum foil as blankets. Kyle asks ICE officers Jeff Corrigan and Dave if the ICE is concerned at all about what they're doing to the kids, pulling them away from their families and making them despise the US government. He explains that their actions are creating a "Mexican Joker" who is going to grow up to take revenge and fight back.
The ICE officers misunderstand this and get very anxious about the prospect of a Mexican Joker and try various methods to prevent that from happening. They put up a puppet show with Mexican Joker raping Princess Star. They even administer electroshock therapy to the immigrant kids, thinking that this "treatment" would help the Mexican Joker overcome his traumatic experience.
At the end of the episode, Kyle and Cartman make many bowler hats out of aluminum foil and scissors and convert the immigrant kids to Judaism in a bid to get them released, as the ICE officers consider imprisoning Jews to be "racial discrimination". However, at that time, Jeff and the ICE officers believe they are experiencing a flashback of the Mexican Joker. They check on the children and see Kyle converting them to Judaism. In a panic that one of them is the Mexican Joker, Jeff shoots and kills his colleagues. He gets ready to release the children in custody. Kyle yells at Jeff that there is, in fact, no Mexican Joker, and tries to explain that he was using an analogy. Jeff, dumbfounded, suddenly blurts out, "I must be in the wrong flashback", runs away down the hall, and drives away.
Following the Mexican Joker incident, there have been changes in the ICE detention centers, such as holding children in separate rooms and allowing American citizens to adopt them. The White family visits the ICE detention center to adopt a boy after their son Jason White was run over by a car. They adopt Alejandro, a Mexican child introduced by Jeff. They later adopt younger Panamanian Padengo Pequeño for Alejandro to look after in a bid to pacify his unhappiness. However, being denied his family for so long and being forced to put on white sunscreen, Alejandro finally snaps and begins to cause havoc while looking like the Mexican Joker. The Whites try to intervene, but the police begin shooting while he makes his escape.
They are of varying appearances but they are all Hispanic with black hair.