To add anyone to your return, you must enter their information in the Personal Info section of TurboTax.
From within your return, click the Personal Info tab, then on that page there is a button to Add a Dependent. Answer the questions from there to add that person to your return.
Please see "What are the qualifications for the Earned Income Credit (EIC or EITC)?" and to determine who would be a qualifying child for the EIC, please see the following TurboTax FAQ:
https://ttlc.intuit.com/replies/4835245