Often, in such fanfictions, Sirius may not trust Harry with certain information about his parents or the wizarding world because he believes Harry is too young to handle it. So, he keeps things from Harry, which can lead to conflicts between them. Harry may feel frustrated that Sirius doesn't see him as a capable young wizard, while Sirius is just trying to protect what he sees as Harry's innocence.
Maybe she wants to humiliate him. By treating him like a baby, she can make him seem weak and powerless.
In this kind of fanfiction, this dynamic can be quite complex. On one hand, Harry could start to internalize the 'baby' treatment and act more immaturely. He might not take risks or stand up for himself as much. However, on the other hand, it could also be a form of protection for Harry. Sirius may be trying to shield him from the harsh realities of the wizarding world. This could lead to Harry having a more sheltered view of things at first, but as he matures, he could break free from this perception and become a stronger character.
One common theme is sacrifice. Sirius is often willing to put his own life on the line to save Harry. For example, in many fics, he battles powerful dark forces without hesitation to protect the baby.
One common scenario could be a character being forced to wear baby clothes as part of the punishment. Another might be having their freedom restricted like a baby, such as being confined to a small area. Or they could be made to eat baby food.
Often, there could be a situation where Naruto has some sort of mental setback. Maybe he forgets all his skills and knowledge, and as a result, the people around him start treating him as if he were a baby. They would then have to reteach him basic things like walking and talking in a ninja - context.
One common scenario is when Harry is in trouble at Hogwarts. Sirius might break into the school in his animagus form to rescue Harry. Another is when Harry is dealing with the mystery of his parents' death. Sirius can share memories or information that he has about that time, like what really happened the night they died. He can also help Harry deal with the Dementors, as he has his own experiences with them.
One possible scenario could be that Sirius Black spanks Harry as a form of discipline for some dangerous or disobedient behavior that Harry has engaged in within the context of the fanfiction's plot.
A third one is 'Sirius's Adventure with Baby Harry'. In this story, Sirius takes baby Harry on little adventures around the wizarding world. They visit different magical places and meet interesting characters. It's a story that emphasizes their relationship and how it grows over time.
Perhaps Draco has developed some sort of complex feelings towards Harry. He could be secretly attracted to Harry and treating him like a baby is a way to get closer to him without admitting his true feelings. Or it could be that Draco has changed as a person and now views Harry as someone who needs his support and guidance, just like one would for a baby.
Perhaps in the fanfiction, Sirius gets involved with the wrong crowd. And Harry, who has a strong sense of right and wrong, can't stand being associated with Sirius any longer. Or maybe Sirius has a falling out with one of Harry's other important people, like Hermione or Ron, and Harry takes their side and starts to hate Sirius as a result.