Reading stories is good for a child's concentration. It requires them to focus on the plot, characters, and details for a certain period of time. This can gradually train their ability to concentrate, which is useful for their study and future work.
A child can also choose based on the difficulty level of the text. If they are just starting to read, simple and short stories with large fonts and many pictures are a good choice. As their reading skills improve, they can gradually move on to more complex stories with longer sentences and more advanced vocabulary.
Parents can set aside a special reading time every day. For example, make it a bedtime routine. This gives the child something to look forward to and makes reading a regular part of their day.
In most cases, reading fanfiction is legal as long as it is for personal enjoyment and not used for commercial purposes without permission. Fanfiction is a form of creative expression by fans, and as long as it doesn't violate any major copyright laws in terms of mass distribution or claiming the work as original, it's okay to read.
It is without a doubt illegal. Laws are in place to protect children and prevent the spread of any material that could potentially harm them or contribute to the normalization of inappropriate behavior involving children. Reading child porn fiction falls under this prohibited category.
It depends on your schedule and your child's interest. If possible, it would be great to read to them every day. Even just 15 - 20 minutes a day can have a positive impact.