To file a separate return for a dependent you need to set up a new account separate from the parents. Online is only good for one return per account. You can use the same email address for 5 accounts. You can probably use the Free Edition or the Free File website
Https://ttlc.intuit.com/questions/1894512-how-do-i-start-another-return-in-turbotax-online
Be sure on the dependent return you check the box that says you can be claimed on someone else’s return. If your only income is W2 and under 12,200 you do not have to file a return except to get back any withholding taken out.
Filing requirements for a dependent
https://ttlc.intuit.com/community/children-dependents/help/do-i-need-to-file-my-own-taxes-if-i-m-a-dependent/00/26111