Prior to the TCJA the kiddie tax was calculated at the parents rates.  (2017 and prior)

When TCJA was enacted the kiddie tax was calculated using the trust rates.   (2018 and after) 

The SECURE Act allows you to now choose to use the trust rates or the parents rates.  (this is retroactive back to 2018)

So by checking the box to use the pre TCJA (i.e. the parents rates) I should be using the same calculations as in 2017.