Each tax return requires a separate account with a separate username and password, but you can use the same email address for all of them. Never start another return by going back and overwriting everything in the return you just filed. This will replace your info, which means you won't be able to retrieve your return later.

How do I start another return in TurboTax Online?

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

If your child has a simple return, you can use the Free Edition: 

https://turbotax.intuit.com/personal-taxes/online/free-edition.jsp