It depends on the amount.
If your child's only income is unearned and doesn't exceed $1,050, it doesn't need to be reported. Capital gain from investments is unearned.
If your child's unearned income is $10,500 or more, he or she will need to file their own return.
If you have other questions about this, ask in the comment section below.
Here is an excellent FAQ that can help you with this topic:
https://ttlc.intuit.com/replies/4790655