How to I file an additional return on my turbo tax account? Thanks

When you use Online TurboTax, you get one return per fee.  Each return needs its own account and user ID.   If you use the same account and user ID for a second return, you overwrite the first one and lose it forever.

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-online/00/25596

Maybe your children's returns can be prepared using the Free Edition if they are very very simple returns.  Or....maybe you would be better off using the desktop software.

 When you use Desktop TurboTax, which you install from a CD or download to your own hard drive, you can prepare and e-file multiple returns for the cost of the software.  You can e-file up to 5 Federal returns—more if you mail the additional ones.  If you have multiple returns to prepare for family members, Desktop is more cost effective.

https://ttlc.intuit.com/questions/1901306-how-many-times-can-i-e-file-using-the-turbotax-cd-or-download

https://ttlc.intuit.com/questions/2750704-what-are-the-advantages-of-the-turbotax-cd-download-software-over-the-online-version