Question

1. There is a girl who is reading a book with legs crossed on the sofa.

2. There is a girl reading a book with her knees folded and raised on the sofa.

3. There are a dog and a cat sleeping on a big coushion.

4. There is a boy playing the piano looking at music.

(Would you check the expressions? Thank you.)

Answer 1

3 and 4 are fine. In 1 and 2, though, you have written what are called "dangling modifiers."

Ask yourself these questions:
1. Are the book's legs crossed...?
2. Are the book's knees folded and raised...?

How can you rephrase 1 and 2 so that these modifiers are clearly describing the girl?