Can I complete a dependents taxes within my account?

To file an additional return with TurboTax Online you would have to start a new account, with a new user id and password, and you would have to pay separately for each one.

If you have several returns to file, the TurboTax Desktop version may be better because you can prepare as many returns as you would like and can e-file up to five for free. Any additional returns would have to be mailed. 

You can also get one free state program with the TurboTax Desktop version, but will be charged to e-file every state return. 

You don't include their income on your return. It would only go on their return if they are required to file one. 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. They can probably use the Free Edition. How to start a new online return
https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-online/00/25596

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 12,400 they 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