It could be an ex - boyfriend of the babysitter. He might be jealous that she's spending so much time at this new family's house and decides to blackmail her to get her attention or to make her leave the job.
Another possibility is that it's a relative of the family she babysits for. This relative might be after the family's inheritance and wants to use the babysitter to get some dirt on the family. So they start blackmailing her to make her do things that would help their scheme.
For the main characters, there could be two teen boys who are always getting into trouble at school. They see the babysitter, a young woman in her early twenties who is very shy, as an opportunity to have some fun. And then there's the babysitter's best friend who sometimes shows up and gets involved in the chaos the teens create. The teens use the babysitter's fear of getting in trouble with their parents to blackmail her.
The plot could start with the babysitter getting strange messages that seem like blackmail threats. These threats could be related to something she did in her past that she thought was forgotten. As the story unfolds, she realizes that the blackmailer is actually someone close to the family she babysits for. She then has to face her past and also protect the family from whatever the blackmailer has planned. All this while dealing with the normal babysitting tasks like feeding the kids, putting them to bed, and making sure they don't get into trouble.
Once upon a time, there was a babysitter named Lily. One day, a mysterious figure left a note threatening to blackmail her if she didn't follow some strange instructions. The instructions were to take the kids on an unexpected adventure in the woods at midnight. Lily was terrified but also curious. She decided to go with the kids. Along the way, they discovered a hidden fairy glen. It turned out the 'blackmailer' was an old hermit who just wanted to show the kids something magical. In the end, it was a strange but wonderful experience.
One possible plot could be that the teens find out a secret about the babysitter, like she failed a really important test in the past but lied about it to her parents. They use this to blackmail her into letting them stay up late and eat lots of junk food. For example, they say they'll tell her parents if she doesn't do what they want.
One plot twist could be that the blackmailer is actually a child's long - lost relative. They use the blackmail as a strange way to get close to the family again. For example, they might ask the babysitter to bring the child to a specific place, which turns out to be a family reunion of sorts.
Once upon a time, there was a babysitter named Lucy. One day, a strange neighbor took a picture of her accidentally breaking a small vase in the house while the kids were napping. He tried to blackmail her, saying he would tell the parents if she didn't do some extra chores for him. But Lucy was smart. She told the parents immediately about the vase and the neighbor's strange behavior. The parents supported her and warned the neighbor not to do such things again.
Perhaps it's a greedy merchant. He has seen Miss Baker receive a valuable gift or inheritance and wants a piece of it. So, he decides to blackmail her by threatening to spread false rumors about her family if she doesn't hand over some of her wealth.
One possible plot twist could be that the person attempting the blackmail is actually being set up by the babysitter. Maybe the babysitter has been planning all along and has evidence to turn the tables on the blackmailer. For example, she could have secretly recorded the blackmailer making threats which would expose their true intentions.
She might initially be scared. But if she's a strong - willed person, she could stand up to the blackmailer and threaten to go to the police right away.
A frequent theme is the sense of power imbalance. The blackmailer often thinks they have the upper hand because they have some 'dirt' on the babysitter. However, the babysitter also has her own strength, like her honesty. For instance, in a story, the babysitter may choose to be honest with the family about what the blackmailer is using against her, and this honesty can turn the situation around. Also, the theme of justice usually prevails in the end, with the blackmailer getting what they deserve.