Welcome, @Geoffrey_De_Oliveir1,
The ability to see subtasks in Calendar view is promised, perhaps within the next quarter.
In the meantime, you can work around this current limitation:
A subtask can be added to be member of the parent task’s project by viewing the subtask in the task detail pane and typing Tab+P (or choose from the Task actions menu) and choosing the parent project.
That will cause the subtask to appear in Calendar while keeping it a subtask.
There’s a side effect: It will now appear in List view and Board view as well. If you want, you could make a “Hidden subtasks” section, move it to the bottom, collapse it, save that view, and above when you add the subtask to the project, then choose Hidden subtasks as the section so it’s out of the way.
Thanks,
Larry