A lady sitting on a bench with a book on her lap with a boy and girl on either side of her.
A woman and two kids next to each other.
A woman has two kids a boy and a girl sitting next to her
A boy and a girl refer to kids. For them to be on either side of the woman requires that the three be next to each other.
There are similar subjects (lady, woman, two kids, boy and girl), doing an action.