How do I do another persons tax return after I have finished my tax returns, i.e., my daughter and son in laws tax returns?

No.  Online accounts are only good for ONE tax return.   Additional returns require a new account with it's own login ID and password.   Any attempt to enter other information in the same account will overwrite and destroy the first return.

The desktop version can e-file 5 Federal tax returns and print and mail as many as you want.

You cannot file multiple returns using the same account with the Online version.  If you use the same account to file other returns that information will overwrite the first return & you cannot get it back.  You would need to create a new account (new UserID & Password) to file their tax return.

The best for filing multiple returns is the Desktop version of TurboTax.  You can file unlimited returns, you get 5 free e-files, any returns after that would have to be mailed.  Deluxe & above includes one free State program, their is a charge to e-file a state return.