If their only income is W2 and under 13,850 they do not have to file a return except to get back any withholding taken out. If they got a 1099NEC for any amount they have to file a return.
Filing requirements for a dependent
https://ttlc.intuit.com/community/children-dependents/help/do-i-need-to-file-my-own-taxes-if-i-m-a-d...
