Just a couple of things that are important, Please follow below steps:
1. add your flows and power app in a solution.
2. build your child flow using manually trigger a flow trigger and add the variables information in the trigger action that you want to get from powerapps.
and in the end, add the "respond back to powerApps or Automate" action and change the connection in run-user specific tab on the flow's property page.
3. when you're building the parent flow, add the action " Run a child flow" this will give you option of flows you can add along with the parameters you might have added in the child flow's trigger. you can Add the parameters from Powerapps directly there.