You use the expression child’s play when you mean that something is very easy to do.
I finished the test very quickly. It was child’s play!
Stop trying to fix the car and call a professional mechanic to do the job. This is not child’s play.
To be a poster child (for something) means to be a perfect example of something.
Mr Cancino went from a poor young immigrant to a powerful businessman in a decade. He is a poster boy for the ‘American Dream’.