A custodial account (UTMA or UGMA*) puts the money in the child's name and Social security number. The theory is that any earnings and capital gains will be taxed at the child's lower tax rate. The child must file a tax return if he has more than $1050 of investment income**. But to cut down on abuse, congress passed the "Kiddie tax" law. Prior to 2018, any income greater than $2100 was taxed at the parent's marginal tax rate. Starting in 2018, the child's income (except earned income), over $2100, is taxed at trust tax rates. See
https://www.marketwatch.com/story/new-tax-law-makes-dreaded-kiddie-tax-more-expensive-2018-09-24
*UTMA - Uniform transfer to minors Act UGMA - Uniform Gift to Minors Act
**If his only income is from interest and dividends, Alaska PFD or capital
gains distributions shown on a 1099-DIV, there is a provision for entering it on
your return, using form 8814. Trading cryptocurrency does not qualify