Your dependent needs to have their own account.
The IRS requires that all taxpayers file a tax return, regardless of age. The Internal Revenue Service requires all taxpayers, regardless of age, to file a tax return and pay the appropriate income tax in any year their gross income exceeds certain levels. This requirement extends to the children you claim as dependents.
Students who are single and earned more than the $12,400 standard deduction in 2020 are required to file an income tax return. That $12,400 includes earned income (from a job) and unearned income (such as from investments or unemployment).
Many students were required to file a tax return in 2020 due to the earnings from unemployment put them over the $12,400 threshold.
If your child is getting a refund it will not matter if they had an extension since you have three years to file and get a refund---there is no penalty for being late if you are getting a refund. If they owe tax, the penalties and interest began on May 17, though, since that is when 2020 tax due was supposed to be paid.
Your dependent can file their own tax return but must say on their return that they can be claimed as someone else's dependent. Dependents are not eligible for stimulus checks.
Your dependent cannot use YOUR TurboTax account since with online you only get one return per fee. Instead of paying to use the software, follow the suggestion above and use one of the Free File options.
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