Adventures in Babysitting

After her boyfriend Mike unexpectedly cancels their anniversary date, Chris Parker invites her friend, Brenda, over to her Oak Park, Illinois house to cheer her up, but is eventually convinced by her mother to babysit the Andersons' daughter, 8-year-old Sara, while they attend a party in Chicago. Fifteen-year-old Brad is originally supposed to go to his friend Daryl Coopersmith's house to spend the night, but he changes his mind when he finds that Chris is the sitter. After receiving a frantic phone call from Brenda, who ran away to the bus station downtown, using all of her money for the cab ride, Chris plans to go alone to pick her up, but is coerced by Brad, Sarah and Daryl to take them with her. On the freeway, the Buick Electra station wagon suffers a flat tire and they are picked up by a kind tow truck driver, "Handsome" John Pruitt, who offers to pay for the tire when Chris realizes she left her purse at the Andersons'. En route, Pruitt gets a call from his boss Dawson with evidence that his wife is cheating on him, and he rushes to his house to confront the infidelity; Chris's mother's car is damaged when Pruitt accidentally shoots out the windshield with a revolver while aiming to kill his wife's lover. Chris and the kids hide in a Cadillac, which is then car-jacked by a thief named Joe Gipp.
Reaching their hideout in the South Side, Joe is confronted for bringing witnesses to their car theft operation and second-in-command Graydon ignores them when they say they won't go to the police. They are detained in an upstairs office and escape through the ceiling, making their way across the rafters and out onto the roof. They are chased by the thieves and enter a blues club where the band on stage won't let them leave until they sing the blues. Chris, Brad, Sara and Daryl recount their events so far that night to the cheers of the audience and are allowed to leave, successfully losing Greydon and his leader Bleak. Meanwhile, Brenda is having trouble of her own; after her glasses are stolen and she doesn't have money for a hot dog, she mistakes a sewer rat for a kitten and is appalled when pest control points it out.
Brad tells Chris about his feelings toward her, but find they are not reciprocated because Brad is two years younger than she is. After separating Daryl from a streetwalker who is a runaway, Chris is reminded of Brenda. They are then found and chased again by Greydon and Bleak, but manage to escape on the Chicago 'L' train and wind up in the middle of a gang fight in which Brad is injured when one of the gang leaders throws a switchblade onto his foot. They take Brad to the hospital, where he receives a single stitch to his toe, while the rest are led to believe he had died. They run into Pruitt, who is now on the lam for his earlier attacks; he tells the kids he replaced the windshield, but they need to pay Dawson $50 for the tire. The kids come across a fraternity house party and Chris meets and becomes attracted to Dan Lynch, who manages to get them $45. He takes them to Dawson's Garage and drops them off. When they find Dawson, his blond hair and sledge hammer leads Sara to believe he is her hero Thor. He coldly denies them their car because of the $5 shortage, but when Sara offers him her toy Thor helmet, he changes his mind and lets them go. Meanwhile, Joe Gipp told Bleak about their troubles and the three are waiting to follow them. The kids find the restaurant where Mike was supposed to take Chris and discover he is there with another girl. Sara slips away on her own to look at a toy store while Chris yells at Mike. Brad stands up for his friend while Daryl kicks Mike into a food cart, ruining the dinner. Meanwhile, Sara is spotted by Bleak, and Graydon chases her to an office building where she goes into hiding on a floor being renovated; the others note her disappearance and follow, accidentally coming across the Andersons' party. After Sara climbs out one of the open windows and slides down the building, Chris spots her and they run upstairs to help.
After pulling Sara from outside the window, Bleak confronts them, but Joe knocks his boss out, giving him a Playboy Magazine that Daryl had stolen, which had important notes that the criminals wanted. The kids hurriedly pick up Brenda from the bus station and rush home, narrowly avoiding the Andersons on the way. Once back home, the kids go upstairs while Chris dismisses Brenda and cleans up the mess left earlier, settling into place just as the Andersons return. Everything back to normal, Chris tells Sara that the night was her last babysitting gig, but all of them agree that it was the best night of their lives (so far) and that Brad and Chris are comfortable just remaining close friends. After Chris leaves, Dan arrives with one of Sara's missing skates. He says he needs a babysitter and is disappointed when Chris said she's retired; he then confesses that the babysitter was for him. Chris decides that retirement can wait and gladly agrees to babysit Dan. With Sara's encouragement (from the bedroom window), Chris and Dan laugh and kiss as Brad closes the blinds.
In a scene after the closing credits, Graydon is seen still stuck on the building ledge.

Question: Whose party did they accidentally come across?
Answer: The Andersons