Copy fields from one table to another with a power automate flow

I have a table with a list of investments, with a one-to-many relationship with a child table, showing balances of such investments. 

The balances table has the contract number of the parent table, a date field, and an amount field. This child table is simply used to show the ebb and flow of the investment over time. 

I need the last balance to show in the parent table, however.

I need a flow that will trigger when a row is added to the child table (that is easy enough), that will copy the date and the amount over to two relevant fields in the parent table. 

Please point me in the right direction. I can't figure out how to do this.