Conduit API as of today, speak about only the projects to which they belong.
When the project they belong is a milestone and finding to which main project they belong to is a tedious process to impossible task.
They are already linked internally, only change is to be added to the api contract which helps to map parents and childs
[ It might be too harsh to say for a display/hierarchy used for tasks, like display for projects. But minimum will help to solve]