That is embarrassing that Support could not understand the question. Which check box - The first one in Screen  3. 

Screen 43  "calculate tax at parent's rate "

Did you check the box in Screen 3?

I did generate a sample return to test this. First thing I notice is that Family Link does not transfer data. Screen 43, you have to input manually, just like in olden days. My guess is that will be updated someday. I don't know your figures, but what I used originally was not high enough to affect a change. I increased the child's interest income to 15,000 and did generate a difference of $60. 

It does work. Not very elegantly and takes more effort than it should, but it does work.

I checked the box in screen 3. I checked the box in screen 43. I manually input the parents data. The child has 6,000 wages and $45,000 unearned income, under age 24 full time student. The program ignores me and calculates the tax using trust rates. Tax year 2018. I am trying to amend the 2018 return to calculate the kiddie tax at the much lower parent's rate. It's not happening.