Did my dependent child (22 years old student) need to file tax returns if he had unearned income of $9,000 (Crypto)?
Yes. A single dependent with unearned income over $2,800 (2021) must file a tax return.
Page 11, https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
Be sure your dependent indicates on their return that they can be claimed as a dependent by someone else.