That's an easy one to research if you just check the 1040 instructions under filing requirements.  

And read Form 8615 instructions, too,  while you're in the virtual library.

Suggest you research pub 929. here is link.

https://www.irs.gov/publications/p929#en_US_2021_publink1000203737

I'll make it easy for you

you must file a return if the dependent has a stock sale 

You don't have to file a tax return just because the child had a stock sale.  However, a return is required when there is a $4,000 gain.  If there is a loss in a "covered" transaction, IRS doesn't care.  

@BobKamman as you said they don't have to file just because they received a 1099B, but pub 501 suggest they do just to ensure they don't get a letter from IRS.

Form 1099-B received.

Even if you aren't required to file a return, you should consider filing if all of the following apply.

In this case, filing a return may keep you from getting a notice from the IRS.

@Terry53029  I know you aren't trying to jack up your fees with unnecessary work, but let's not encourage those who do. As the IRS Pub says, you might want to file a return if

• Box 1e of Form 1099-B (or substitute statement) is blank.

That's why I said the loss would have to come from a covered transaction.  You can't have one of those with a blank Box 1e.  And please don't misquote me by saying "you said they don't have to file just because they received a 1099B."  

@BobKamman Sorry if you think I misquoted you, as you actually said  "You don't have to file a tax return just because the child had a stock sale". To me that is same as saying received a 1099B. At any rate I don't charge my clients for their dependents return whether they are, or not required to file.