To file a separate return for your dependent you need to set up a new account. Online is only good for one return per account. You can use the same email address for 5 accounts.
Https://ttlc.intuit.com/questions/1894512-how-do-i-start-another-return-in-turbotax-online
Be sure on their return they check the box that says they can be claimed on someone else’s return. If their only income is W2 and under 12,000 they do not have to file a return except to get back any withholding taken out.
Filing Requirements https://ttlc.intuit.com/replies/3302274
