There are some help articles https://proconnect.intuit.com/community/estimated-taxes/help/using-form-8814-or-form-8615-to-report-... has broken links that @IntuitBettyJo will put the content team on to clean up. 

To answer your question - You are in the Parents income screen, look up and a bit to the right and you will see Child - Click that and you will given a screen to add as many tabs as you need to add other childrens info and income.