EDIT:
Parent - Child flows are only available if they are created inside a solution. Known as "solution aware flows".
Create the child flow
Type: Instant flow - Manually trigger a flow
Add one or more "Choose the type of user input"
Name them appropriately
The values are retrieved from dynamic content selection box.
Always add action "Respond to PowerApp or flow" at the exit points of your flow.
The parent flow will wait for this reply or timeout error if the child flow never responds with this action.
After child flow is saved..
Create the Parent flow.
When using action "Run a child flow" the input parameters are exposed
and responses from the child flow can be retrieved from the dynamic selection list.
Bonus tip:
Currently json objects cannot be passed between Parent and Child flows.
To get around this convert json to string using: string()
Then add action Parse JSON in the other flow to convert it back to json.
