I'm pretty sure the information isn't used to calculate the tax rates though. The TCJA changes made it so the tax rate was based on trust and estates, no? I assumed that was the whole point of the "Do you want to use pre-TCJA rules or not" question. If you used pre-TCJA rules, then it's taxed at a parents rate, but otherwise it would be taxed at the rate of trusts and estates.
https://www.irs.gov/taxtopics/tc553
"If your child's interest, dividends, and other unearned income total more than $2,200, it may be subject to tax. The unearned income of certain children is taxed using the tax brackets and rates for estates and trusts unless an election is made to calculate the child's tax based on the parent's tax rate."
