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'.
Research by scientists based at King's College London and Brown University, Rhode Island, suggests that immersing children in a bilingual environment before the age of four gives them the best chance of becoming fluent in both languages. They studied 108 children with normal brain development between the ages of one and six.