MY DEPENDENT HAD A JOB
If your dependent has a W-2 for his after-school job, summer job, etc. you do not include the information on your own return. You can still claim your child as a dependent on your own return. He/she can file his own return for a refund of some of his withheld wages from boxes 2 or 17 (he won’t get back anything for Social Security or Medicare), but MUST indicate on it that he can be claimed as a dependent on someone else’s return. (Supervise this closely or prepare it for him!)
If your dependent’s earnings were over $400 and were reported on a 1099Misc or 1099NEC then he must file a return and pay self-employment tax for Social Security and Medicare.
You might also want to use free software from the IRS Free File versions:
https://apps.irs.gov/app/freeFile/
The standard deduction amount for an individual who may be claimed as a dependent by another taxpayer cannot exceed the greater of $1300 or the sum of $450 and the individual’s earned income (not to exceed the regular standard deduction amount of $14,600).
The options are the standard deduction and itemized deductions. Your dependent would not likely have expenses to itemize, therefore the standard deduction would be the one to take. Itemized deductions include medical expenses, charitable donations, mortgage interest, property taxes. So since your 15 year did not likely pay them in an amount that was greater than her standard deduction, then she would take the standard deduction.
It would be incredibly rare for a 15 y.o. dependent to itemized deductions. The $14,600 (maximum) standard deduction usually wipes out any tax from a teenager's part time job. If he has income on a 1099-NEC, he gets to take his "business" deductions on Schedule C. Schedule C deductions are in addition to the standard deduction, not instead of.