Simple ... open a new account for them using a different userID and password from your account ... do NOT try to use your online account again for their return. AND since they are your dependent they MUST indicate they will be claimed as a dependent on their return.
If they qualify you can use one of the 8 IRS FREE FILE options ... for 2021 your AGI has to be $73K or less ...
https://www.irs.gov/filing/free-file-do-your-federal-taxes-for-free
Or use the TT free option : https://turbotax.intuit.com/personal-taxes/online/
