Stand-In Parents

This trope is common in sitcoms and animation.

There's a father/child or mother/child event coming up, and the kid is either embarrassed of how lame their parent is, or knows their parent is not capable of doing the event and will lose it for them -- again—if they come. Or they're afraid if their parent comes to the event, the kid will get in some sort of trouble.

So they find someone else to pretend to be their mom or dad.

The gambit usually fails, and the outcomes tend to fall into three predictable categories:

The parent finds out, is hurt that their child thinks so little of them.
The parent finds out and makes a disastrous attempt to be what their child wants, and that brings them closer because the parent cared enough to try for their child's sake.
The parent never finds out (or not until after the fact).

An Aesop about being honest and loving someone just the way they are is usually intended.

The other main variations are:

when the character is not really a child but pretending to be one for whatever reason
the character looks like a child but is Really Seven Hundred Years Old
the character is an orphan

...in which case they need stand-ins for parents who don't or may have never existed.

Related to Paid-for Family.

Examples of Stand-In Parents include:

Anime and Manga

In the second Magical Girl Lyrical Nanoha Sound Stage, Fate's surrogate mother figure Linith asks her biological mother Precia (who generally treats Fate like trash) to act as a real mom to Fate for just one dinner. Yeah, Fate had a crappy childhood.
Miu does this once in Kenichi: The Mightiest Disciple. When she applied to the high school, she gave false data about her parents (including the lie about them being still alive), because she wanted to present herself as a girl with a normal family. This backfires on her when the representatives of her school come to Ryouzanpaku to check up on her. Hilarity Ensues when she eventually chooses the jerky (although good-hearted) Sakaki and the Lady of War Shigure with No Social Skills to pose as her parents.

Literature

In Apt Pupil, by Stephen King, Dussander attends a parent/teacher conference at Todd's school, impersonating his grandfather, so they can manage to keep Todd's parents from finding out that his grades are in free-fall.
In Mercedes Lackey's Jinx High, the villain who BodySurfs to her daughter each generation appears to be under 18, and has an artificial construct that masquerades as her aunt/guardian while her mother is in a mental hospital with the personality of the daughter just evicted from her original body in Mom's body.

Live Action TV

Saved by the Bell: In one episode, Mr. Belding wanted to have a conference with Zack's father, as his mother proves to be far more forgiving of his misdeeds. To keep his work-alcoholic dad from finding out about, Zack gets a waiter friend at the Max to pretend to be his father instead. (Since that guy was an out-of-work actor he decided to ham it up.) His dad actually finds out about the conference, so Zack's waiter friend pulls double duty as a stand-in for Belding. The gambit naturally fails when - despite Zack's best efforts - Belding and Zack's dad run into each other at a school function.
The short-lived show Sons Of Tucson was based on this. Three brothers hire a fake dad to keep up appearances after their real father is put in prison.
In Ghost Whisperer the children pretended to be their own mother after she had died, for fear of social services splitting them up on finding out they had no parent or guardian.
Kenan once felt the need to hire stand-ins for a conference with his school principal on an episode of Kenan and Kel, not wanting to tell his parents that they were called in by the school to meet the principal and knowing that the principal has no idea what his parents look like. He settles on a classically trained actor and a total ditz who says she's seen a lot of movies and thinks she can wing it. Unfortunately, The Ditz ruins everything by not remembering the name of the person she's supposed to pretend to be and inviting the school principal to their home for a family dinner, which the principal accepts, requiring Kenan to get his real parents to leave the house for a night so he can stage a dinner with his mock parents. Hilarity Ensues. The real kicker, however, was that the reason for the meeting with the principal was to discuss how great Kenan's grades and schoolwork was.
On Psych Shawn gets his uncle Jack to stand in for his dad at school. Shawn's dad finds out and is irritated rather than hurt. Especially since Jack really is cooler than he is.
Good Luck Charlie Gabe cons PJ and Teddy into standing in as his parents for a parent-teacher conference so their real parents don't find out.
On That's So Raven, Raven posed as her own mother at a parent-teacher conference to prevent her mother from finding out how bad her grades were. When the mother (ineviatably) found out, she was both angry that Raven had lied to her and offended because of the insulting fat suit Raven had used for the disguise.
Malcolm in the Middle: Reece finds some incriminating letters on his neighbour's old laptop. He uses this to blackmail the neighbour into doing his bidding. His last act is to get the neighbour to pose as Hal at a meeting with the principal. After this, the neighbour now has material to use against Reece, and he is certain that Reece is more scared of Lois than he is of his wife.

Western Animation

Jimmy Neutron: Cindy Vortex had an aunt stand in for her mother at Retroville's parent/child sporting day.
Fairly Oddparents: Timmy Turner used a wish to try out his friends' dads because his own dad was so lame.
- He also had Cosmo and Wanda stand in as his parents, and hilarity ensued.
Kick Buttowski: Kick goes around the neighbourhood trying to find someone to act as his dad for "bring your father to school" day. His father finds out and is hurt that Kick asked Wade to pretend to be him. He then shows up and tries to be cool, with unfortunate results. They end up meeting each other halfway.
Dexter's Laboratory: Dexter uses Mad Science to make Dee Dee impersonate his mother for a parent-teacher meeting.
- Then again when Dexter tries to create his vision of a "perfect father" because he didn't want to bring his dad in for Parents Day.

In an episode of Daria, Tiffany hires a model to act as her mother in a mother/daughter beauty contest.
Avatar: The Last Airbender has Sokka and Katara stand in as "Kuzon"'s parents Wang and Sapphire Fire, when he gets into trouble at school.
Invader Zim has robots to stand in as his parents, and despite them being horribly malfunctioning, nobody notices.
Phineas and Ferb plays with the trope. Mr. Fletcher after stumbling into Perry's lair and getting mindwiped, is replaced by Agent P operating a robot duplicate of him for Candace's father/daughter picnic and competition.
Recess Spinelli hires a biker couple to pretend to be her parents because of how incredibly dorky and humiliating her real parents are. This backfires on her when the couple desides to blow off her cover by taking her money by force in front of everyone.