This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / The Baby-Sitters Club
The death of Louie; Mimi's stroke, deteriorating health and eventual death; and a lot of Abby's autobiography.
Louie's death is particularly bad because he doesn't simply die of old age. He had to be put down. Readers went through several books seeing Louie the collie as an energetic, happy, beloved family pet and see him slowly become weak and sick to the point where the Thomas-Brewers admitted that they basically had to put him to sleep, because they couldn't bear for Louie to be in such pain. There's tons of little moments throughout, too, like David Michael trying to keep a half-blind Louie from going down stairs and getting hit in the eye for his trouble, David Michael opting to sleep downstairs with Louie during his last night home, the funeral...
Speaking of Mary Anne, just the fact she grew up with a dead mother, had to contend with several Mother's Day projects, her Father's overprotective-ness, her relationship with Logan, her attempts to be a very good girl, she loses a (non-BSC) friend to a freak accident, she can be the butt of some jokes due to her meek nature, and her house burns down.
She even later finds out there was some contention between her father and maternal grandmother, which produces two more Woobies, the father who lost his young wife and is trying anything to avoid losing his daughter and the grandmother who fears losing her grandchild after the loss of her daughter and later her husband.
Not to mention that she starts going to therapy in later books and it's strongly implied that she's depressed.
Jessi's baby brother, Squirt, getting hurt in a car wreck.
Stacey having to choose between living with her mother or father. She knows one of them will be hurt either way.
Claudia: despite being artistic, having a creative take on her clothing, reading Nancy Drew novels, a good caregiver, displaying good sleuthing in the Mystery books, and keeping her head together in very dire situations thinks less of her intelligence since she isn't as studious as her older sister.
Kristy's breakdown after it's revealed in "The Truth About Stacy" that the Babysitters Agency deliberately sabotaged the BSC by sending two lackeys to skip out on sitting jobs which made the BSC look bad. Kristy's in tears, sobbing about how she's always tried to just do the best job she can and she's heartbroken that someone would be that malicious for no real reason.
There's something about the series' concluding that hits hard: as cheesy, unrealistic and downright irritating as the books could be at times, waving goodbye to these characters feels like leaving old friends behind. Or maybe that's just the nostalgia value.
Nate and Joey from Claudia and the Terrible Truth, who are undergoing their dad's abuse when Claudia starts sitting for them. Claudia observes evidence of both verbal and physical abuse, but fortunately is able to get the boys help through her mom; Nate and Joey's mom then leaves with her kids.
Victoria from Mary Anne and the Little Princess. She comes off as snobby but is really lonely because her parents are never around due to their jobs at the U.N. and royal status. Mary Anne helps Victoria confront this and she gets better.
Jenny Prezzioso is a Bratty Half-Pint extraordinaire, but you have to feel sorry for her considering all the attention her parents lavish on Jenny's baby sister Andrea. In Mary Anne and Miss Priss, this was carried to the point that Mrs. P was ignoring Jenny in favor of baby Andrea's modeling career, and Jenny felt the only way to get attention was to try modeling, too. Double Tear Jerker because the gambit didn't work.
Danielle, the little girl with cancer in Jessi's Wish.
In Dawn and Whitney, Friends Forever, readers meet Whitney, a twelve-year-old girl with Down Syndrome. She and Dawn become friends, but Whitney breaks down in angry tears when she realizes that her parents hired, and pay, Dawn to babysit her.
In Kristy and the Worst Kid Ever, the Papadakises take in Lou, an extremely angry and rebellious foster child. It's revealed over the course of the book that she's devastated at losing her birth family (her mother abandoned her, and Lou was subsequently separated from her beloved brother, who has also been taken into foster care) and is constantly tense and unsettled because she's been sent from one foster home to another.
Lou pops up again in Abby and the Best Kid Ever. As the title indicates, this time she is well-behaved, to the point that it seems overdone and forced. Abby eventually finds out that Lou is terrified her new foster parents will send her away if she makes any mistakes.
During Aloha, Baby-Sitters, Claudia's entire arc is a Tear Jerker. During a trip to Pearl Harbor, Claudia struggles with her identity as a Japanese-American, and the fact that the Japanese were America's enemies in World War II. She even assumes a veteran she meets will hate her he doesn't; as a matter of fact, he's the one who makes her feel better about being Japanese.
A subplot taking place over several books was Claudia's aunt Peaches trying for a baby. After several failed attempts, she finally gets pregnant - only to lose the much longed-for baby. This was eventually happily resolved with Peaches having a healthy baby.
The whole of Mary Anne and the Memory Garden, in which Mary Anne's friend from school is killed by a drinking driver.