A man sitting on steps with a child as a bystander walks by.
A man and a child sitting on steps saw a bystander.
"A man sitting on steps with a child as a bystander walks by" is a rephrase of "A man and a child sitting on steps saw a bystander."
There is both a man and a child sitting on steps.
If a bystander walks by, it can be inferred that they saw a bystander.