Hey @Kirk_Jess welcome to the community 
A workaround in the meantime would be adding the subtasks also to the project as it is also explained in this thread: https://forum.asana.com/t/sub-tasks-in-timeline/117604/2?u=andrea_mayer
The only downside is that it would not show directly under the parent task in the timeline view.
In the meantime I recommend to upvote here: Timeline - Schedule Subtasks
@Rebecca_McGrath I think this thread can actually be merged into the other one 