The Kids Are American

A movie takes place in a foreign land. Just to prove it to you, all the adults have an accent; one may go so far as to be the Funny Foreigner. Then a kid shows up speaking fluent American English, and unless the kid is a stellar actor, this can poke a hole in Suspension of Disbelief.

This actually has a fairly logical explanation. The vast majority of Hollywood's movies are filmed in California, and there are a number of laws involving child actors all around the world. Finding a kid who lives in or near Los Angeles just makes the situation a lot less complicated.

Truth in Television if the family has immigrated; children's speech patterns aren't fully set until age 12 or so, anyone younger than that can learn a new language without an accent with sufficient immersion.

Film (Animated)

 * Robin Hood: Most of the adults have British accents, but all the kids have American ones.
 * This is actually consistent. The British accents are only on the upper-class characters. The lower-class characters have American, usually Western, accents. All the kids we see are lower-class.
 * In How to Train Your Dragon, every single member of the parents' generation speaks with a Scottish accent. Every single member of Hiccup's speaks with an American one.
 * Aladdin, if the lead pair count as kids when they're young adults.
 * Everyone in Aladdin sounds either American or British. Nobody has an Arabic accent at all.
 * Exceptions: The stereotypical Arab who sings the Framing Device opening song, and Jafar's sacrificial Mook, Gazeem.
 * There's also a few villains from the second and third film that have Arab sounding accents.
 * An American Tail: The Mousekewitz family starts off in Russia, and while Mama and Papa have extremely thick Russian accents, Fievel and Tanya have American ones before they move to America.
 * In Pinocchio, Geppetto (Christian Rub) has a foreign accent but Pinocchio (Dickie Jones) has a Midwestern American accent.
 * Disney has also inverted this trope, however. In Peter Pan, the Darling kids are practically the only characters with British accents. The same is true with Alice in Alice in Wonderland, though to a lesser extent. Some Wonderland characters sound English, but the most prominent ones sound American. (Incidentally, Alice and Wendy were voiced by the same British child actress, Kathryn Beaumont.)
 * In The Aristocats, Duchess sounds Eastern European (voice actress Eva Gabor was Hungarian), while the kittens sound American, despite being natives of Paris, France.

Film (Live Action)

 * The Sound of Music: Everyone has a British accent, except the youngest kids in the von Trapp family.
 * Averted by Indiana Jones and the Temple of Doom. Short Round has an Asian accent.
 * In the 1971 movie version of Charlie and the Chocolate Factory, Charlie and his family are a few of the only ones with American accents — and in the 2005 version, they're a few of the only ones with British accents.
 * Inverted in Aliens: Everybody's American, but the one kid sounds British, and the film was made in Buckinghamshire in the United Kingdom. Justified, since she's unrelated to any of the adults that appear on-screen.
 * Also, the actress was American but raised in the UK.
 * Lampshaded: The DVD interviews note that the actress's natural English accent is noticeable in some scenes (such as "There's a short-cut across the roof") but it was kept in the finished film.
 * We also don't know what the population make up of the colony was. There might have been lots of Britons there.
 * Indeed, the extended edition DVD shows Newt's family, and they all have mild British accents.
 * In Jurassic Park, John Hammond is Scottish while his grandchildren are American. However, his company is apparently U.S.-based, so it's easy to imagine Lex and Tim were raised in America. But then in The Lost World, American actor Arliss Howard plays Hammond's adult nephew with an English accent...

Live Action Television

 * Inverted with Queer as Folk: The American series was set in Pittsburgh, but filmed in Canada -- the kid has a Canadian accent.
 * Similary inverted in Power Rangers Samurai, in which all the adults have American accents, but flashbacks of the Red and Gold Rangers have them as kids with New Zealand accents.
 * A minor example from Lost: (Scottish) Desmond and (English) Penny's son, in one of his few lines, says "Mommy" instead of the British "Mummy". They're filming in Hawaii and can't exactly get a British 3-year old, so it gets a pass.

Western Animation

 * Avatar: The Last Airbender is a good example: None of the main, 12- to 16-year-old actors have non-American accents, but plenty of adults do: Iroh, played by Mako, has a Japanese accent, as does the warden from the episode "Imprisoned", played by George Takei. James Hong plays the mayor of Chin in season two, which also introduces the Foggy Swamp Water Tribe, whose members sound as if they hail from the Deep South. This is probably a Translation Convention, as they all write in traditional calligraphy.
 * On the other hand, Sokka & Katara's, Toph's, and Zuko's parents all have American accents. Worth noting, however, is that Aang's Parental Substitute Gyatso has a Japanese accent.
 * It's also worth noting that with a larger talent pool of adult actors the show is far more likely to cast Asians for the adult roles, even if they have American accents, like Daniel Dae Kim as general Fong or George Takei as the warden.
 * Intentionally done in Gargoyles. The elderly Hudson has a thick Scottish Accent while the rest of the clan range from American Accents to neutral. This was done to highlight the generational differences and acceptances each had of the modern world. As the series progressed, more Clans living in Modern Times would be discovered (Specifically the English, Japanese, and Guatemalan clans) spoke in region appropriate accents. An explanation of the Avalon Clan (who all grew up with Scottish or English accented characters yet still spoke with an American accent) was never given.
 * Jackie Chan Adventures: Both Jackie and Uncle have been living in America for years, but retain noticeable Chinese accents. Jade is fresh out of Hong Kong, but sounds like she grew up in Los Angeles.
 * Some of the younger locomotives in Thomas the Tank Engine were given American accents in the later episodes, while some of the older locomotives were given British accents.
 * Averted with the humans however, where both the children and the adults in that case all have British accents.