Translation:El niño está en su cuna.
Why is it esta and not es
This sentence is about the boy's location. Location is usually expressed by some form of "estar."
Explanations and examples at studyspanish.com (Grammar Unit Two)
why is el chico wrong?
I have been asking myself the same question. Maybe it's because the word "chic@" refers to a child that is a bit older, one that doesn't need a crib anymore? (-;