I need to set up a tax return for my grandson using my Turbotax account. Or, do I need to set up another account? If so, how?

You can only prepare one tax return per Online account.  If you need to prepare a return for someone else, you will need to create a new TurboTax Account.  On the TurboTax log in screen, click on "New to Intuit? Create an account."

can i file my grandsons taxes on my turbotax account

NO!  Online is only good for one return per account.  But you can use the same email address for 5 accounts.

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