The SECURE Act included a provision that allows parents to elect to tax the unearned income of their dependent children at the parent's tax rate. This provision is retroactively applicable for tax year 2018.
The 2018 Publication 929 is not being revised at this time. Instead, see the 2019 Publication 929 for more information. If you want to make this election for tax year 2018, you will need to file an amended return on Form 1040-X. You can find more information about amending a tax return at IRS.gov/form1040X.