Welcome to the Community Forum @Amanda_Pruden!
Sorry for the trouble with the subtasks. You are correct, as subtasks do not inherit the Project from the parent task , the only way for them to show in your project Calendar view, for now, is by manually adding them to a Project.
This is a very popular request in the Forum and generally speaking among our users; we’re fully aware this is something we need to improve and while I don’t have an update to share at the moment, this is definitely on our radar. I recommend you upvote for similar features in these threads:
I hope this helps!