A lady sitting on a bench with a book on her lap with a boy and girl on either side of her.
An adult and two children are on a rollercoaster.
Either, the people are on a bench or on a rollercoaster, not both.
Two children could be two boys or two girls instead of a boy and a girl.  If a lady, a boy and a girl are sitting on a bench they can not be on a rollercoaster.
A bench is not the same as a rollercoaster.