First, think about the characters you want to focus on. You could pick your favorite from the Baby Sitters Club or create a new character who interacts with the existing ones. Then, come up with a plot. It could be something simple like a day in the life of the club with a small problem to solve, like a lost toy during a babysitting session. Next, write in a style that matches the original series. Use language that is appropriate for the age group and keep the tone positive and friendly.
When writing 'Baby Sitters Club Fanfiction', you need to consider the setting. The club usually operates in a small town environment, so you can use that to your advantage. You can describe the familiar places like the school, the park where they often take the kids, and the homes of the families they babysit for. In terms of plot, you could introduce a new family moving into town and the challenges the baby sitters face in taking care of their kids. It could be that the new kids have different cultural backgrounds or hobbies. Also, make sure to have good dialogue between the characters. The conversations between the baby sitters should feel natural and reflect their different personalities.