This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Baby-Sitters Club
Claudia sits for a family in which the husband abuses the kids. After Claudia and her mother rescue the children and their mother, Claudia gets a phone call late at night. It's the abusive father, wailing at her to give him back his children.
In one of the early books, a serial thief was breaking into houses in nearby towns, making calls to prospective targets and instantly hanging up if anyone answered. The club starts getting calls where the caller hangs up once they answer, and later in the book, Claudia's babysitting and realizes that there's a prowler outside. It's one of the guys from her class, being a moron, and the calls were from another guy who couldn't work up the courage to ask her out, but it's a pretty Adult Fear-ridden situation.
Then Dawn's brother Jeff (both are stranded) theorizes they could be in Nova Scotia for all they know and another charge refers to the book Baby Island.
And Dawn and Jeff's mother being terrified that even if the kids are found alive, her ex will sue for custody because of the whole incident. When you consider how many perfectly decent and good parents have lost custody for less, it's a very legitimate fear.
Stacy and the Haunted Masquerade is far scarier than most of the mysteries in that the villain is a legitimate escapee from a mental institution who is revenge-crazed.
One of the sitters from the rival Babysitters Agency permitted Jamie, age 4, to go outside in the winter without gloves, hat or scarf and didn't bother to supervise him, resulting in him playing in the street. The BSC manage to get him kitted out and safely in the backyard but all of them are horrified at what could have happened if they hadn't come by.
Mary Anne Saves The Day features a little girl coming down with a high fever, imagine what could've happened if Mary Anne and Dawn weren't clear-headed and capable enough to take care of her.
How about this bit in Dawn and the Impossible Three where Dawn wasn't informed that a charge is allergic to chocolate? If Mallory hadn't swooped in and took the treat, there could've been a dead two year old...
And more! When everyone fears that Buddy Barrett was kidnapped only to turn out it was his father taking him out for the day. Scary since real-life divorced parents might let their ill feelings towards their spouse take over...