Some celebrities who appeared in the live-action segments of The Super Mario Bros. Super Show! did so because their children were huge fans of the games.
This could be considered to be the main career goal of Simon Pegg now. After his daughter, Matilda was born in 2009, he did stuff like Ice Age: Dawn of the Dinosaurs, The Chronicles of Narnia: The Voyage of the Dawn Treader and The Adventures of Tintin (2011). The latter also starred his comedy partner (and best friend) Nick Frost, who might also be doing this now, as the release dates of Ice Age: Continental Drift and The Adventures of Tintin (2011) coincided with the birth of his son. He also made a few guest appearances on Phineas and Ferb since he and his kids were fans of the show. In the case of Ice Age, both Pegg and Frost also count as Promoted Fanboy, as they were fans of the previous movies before being in the third and fourth films respectively. The Promoted Fanboy thing also counts for Queen Latifah and Josh Peck, as well, since they were fans of the Original before getting cast in the sequel. (Pegg has also carried this over into live-action work as he's now most widely known now for playing Scotty in the recent Star Trek films.)
Ray Romano also claimed that he took the role in the first Ice Age movie because his children were too young to understand his other work at the time. Ray has also said he liked having the first movie to use as a threat towards them as well, as in, "We won't go to the premiere if you don't behave." Interestingly enough, his kids provided voices in Ice Age: The Meltdown.
Denis Leary also has admitted in many interviews that he only does the Ice Age franchise in general to do something his nephews and nieces could watch with him in it. He even said, "My nieces and nephews think the only thing I do is Ice Age."
J.Lo also said she was happy to be in something her kids could watch while promoting Ice Age: Continental Drift.
Jack Black was in Kung Fu Panda so his kids could watch him in something child-friendly (although he has done numerous family-friendly films and TV series).
It may not be why she took the part, but Angelina Jolieclaims that her playing Master Tigress is the only reason her kids think she's at all cool.
Gary Oldman in Kung Fu Panda 2 - where he gets to be both a villain and in a movie his three kids can watch!
Ben Stiller was in Madagascar so his daughters could watch him in something appropriate. Chris Rock's motive for doing the film was part this, part Awesome, Dear Boy, as he wanted to not only be in something his daughter could watch, he also wanted the unique opportunity of having the chance to buy her a stuffed toy of a character he was playing. Ben also appeared in Megamind and Turbo for the same reason. Interestingly enough, both of his daughters played schoolkids in the former film.
Will Smith actually said he did Shark Tale to have something his kids could watch as well.
A large part of why Rihanna did Home (2015) was for her younger fans and for young black girls to have an animated character to call their own.
Also from the movie, David Hasselhoff agreed to appear in the film before even learning what role he was being offered because his children loved watching the show so much.
Tom Kenny's role as the titular character could be considered this. Before he did voice-acting, he was a comedian, but when his son Mack was born, he decided to voice the lead character. He also did work on Rocko's Modern Life for the same reason, and it also could be why his career mostly shifted to voice work in cartoons, though he would still occasionally focus on his comedian roots.
Former Skid Row frontman Sebastian Bach appeared in the Clash Of Triton special as the titular character for his then 12-year-old son London and his then-three year old daughter Sebastiana.
Dennis Quaid guest-starred on the show as the titular character of "Grandpappy the Pirate" for his three children, who all loved the show.
Ray Liotta guest-starred in the episode "WhoBobWhatPants" for his nine-year-old daughter Karsen. He also appeared on Hannah Montana for the same reason.
English singer Victoria Beckham guest-starred in the Clash Of Triton special as the wife of King Neptune, Queen Amphitrite because her three sons love the show.
Clancy Brown has cited his daughters as at least part of the reason he took on the role of Mr. Krabs.
Up until probably season 8 (or thereabout) of The Simpsons, pretty much the only way the writers could get their idols of 50+ years on the show was if they had kids or grandkids who watched the show.
Nancy Cartwright (who voices Bart) actually said in an interview that after Meryl Streep recorded her lines for her guest spot in “Bart’s Girlfriend”, she said: "My kids haven't seen many of my films but this is something I think they will watch". And then she asked for Cartwright's autograph. Yes, really.
Willem Dafoe appeared in the episode "The Secret War Of Lisa Simpson" for his teenage son Jack.
Notoriously reclusive author Thomas Pynchon made three guest appearances on the show in part because his son was a big fan of the show.
Dennis Weaver guest-starred on the show so his grandkids, who were teenagers at the time, could see him on TV. This could also explain his role in Home on the Range.
All of the celebrities on Teen Titans Go!, save for two of them, fall under this category, as they play roles on the show if their children fall in the target demographic or love comic books and superheroes:
Fall Out Boy appeared on one of the specials because Pete Wentz' three sons were big fans of the show. There's also the fact that Pete had become a fan by watching with his kids, to the point where he asked Tara Strong if she could let the band to guest star in an episode following a very perfectly timed encounter.
LeBron James also appeared on the show for his daughter Zhuri.
Will Arnett played Slade in Teen Titans Go! To the Movies because his kids were big fans of the show. According to an interview he did for the Teen Titans Go! movie, he said that he likes to make films that his children can enjoy to enrich the movie-going experience.
Nicolas Cage played Superman in the film for the same reason, as his son Kal-El is a big fan of comic books, and Cage wanted to make a movie they could watch together.
Jimmy Kimmel played Batman in Teen Titans Go! To The Movies for his two children, Jane and William. He also played Tim's dad in The Boss Baby and did a segment on his show with the cast of Sesame Street on his own show for the same reason.
Jenna Dewan starred on Blaze and the Monster Machines because her 5-year-old daughter loved the show and wanted to see a fan-character she once drew based on the show for her mom (which inspired Dewan's character Bunny) come to life, making this also a case of Official Fan-Submitted Content.
Robert Downey Jr. took the role of Lois's brother (The Fat Guy Strangler) on Family Guy (his only animated work to date) because his teenage son was a fan of the show.
Hilary Duff guest starred as the Ice Witch in an episode of Dora the Explorer for her then-one-year-old son Luca.
Taron Egerton says part of the reason he took the role of Johnny in Sing is that a lot of his films aren't suitable for his younger sisters. (The other part was Awesome, Dear Boy; they said "Do want to play a singing gorilla?" and he said "Yes, obviously!")
Colin Farrell, Beyoncé, and Pitbull did Epic (2013) for the same reason; in the latter's case, it was because he wanted to be in something his boys could see.
Deedee Magno Hall voices Pearl in Steven Universe for this reason. Evidence appears to be her going to the 2014 SDCC in the title character's iconic pink and yellow colors. It was a gift from her goddaughter. The show's creator, Rebecca Sugar, has stated in interviews that she wanted to make the kind of show she'd like to have watched as a kid, and a desire to create something the kids watching could be inspired by.
Trey Parker, co-creator of South Park, accepted the role of Balthazar Bratt in Despicable Me 3 because he had never been asked to voice a project that he wasn't personally involved with or did through a mutual friend, and it was something he could show his three-year-old daughter.
"I can't wait to see her reaction when she hears my voice come from Sid Fishy's mouth."
Wanda Sykes played a witch in the episode "Bubble Puppy's Fin-Tastic Fairy Tale" because of her daughter Olivia, who was three at the time. She also starred in Ugly Dolls for the same reason.
A variation happened with Animaniacs. Mindy and Buttons were originally written out of the show by Steven Spielberg, but his daughter saw his drawing of the characters and loved it so much that he kept them in.
Alex Trebek played a game show host on an episode of Rugrats so that his son Matthew, who was three at the time, could hear his dad's voice in a show aimed at children.
This might explain Alexa and Carlos PenaVega's roles in The Loud House as Carlotta Casagrande and Bobby, respectively. The latter actor took his role in the show after their son Ocean was born.
Much like what happened to James Corden's Deleted Role in the Teen Titans Go! movie, Madonna wanted to play Dil in the Rugrats movie for her children, but got replaced with Tara Strong in the final movie because of how accurate her scratch voice for Dil was to an actual babynote so much so that, allegedly, a new mother in the room actually started lactating. She would later get an actual kids' role in Arthur and the Invisibles.
Fred Willard began doing voice acting when his grandson was born, beginning with The Spooktacular New Adventures of Casper. These roles even continued after his grandson grew up, with his most notable role being Pop-Pop in The Loud House.
Mel Winkler starred in Oswald as Johnny Snowman for his granddaughters.
Chris and Martin Kratt said they produced Wild Kratts so there would be something of theirs for their children to watch note Technically, their other PBS shows fit the bill, but Wild Kratts is faster paced, is geared toward older children, and has a higher episode count than all their other shows combined., adding that its production was less demanding of their time than other material they produced. Some of their children had recurring roles on the show as well.
Beyoncé guest starred as Shine in Wow! Wow! Wubbzy! because her nephew Daniel Julez Smith (son of Solange Knowles) was a fan of the show.
Many of the celebrities who are taking roles in PAW Patrol: The Movie movie are starring in it for their children who are fans of the source material, including Jimmy Kimmel, Kim Kardashian, Tyler Perry and Randall Park. Kardashian even arranged an early screening for her children and a few of their friends, buying them merchandise for the movie.
Similar to Fred Willard, David Lander played roles in various cartoons after the birth of his daughter Natalie beginning with Galaxy High and took various voice roles even after his daughter grew older (though one of them might be because she worked on another show owned by the same channel.)
Larry King starred on Arthur for his then two-year-old son Cannon.
Gary Sinise said he did Open Season because his kids had been after him for years to do an animated film. Inverted in that by the time he gave in, they were 14, 15, and 17...and had been watching CSI: NY for a while.
that her playing Master Tigress is the only reason her kids think she's at all cool.