It won’t work as you proposed because anyone (your clients) who can see the parent task can see all subtasks.
It could work the other way around–you could designate a single subtask of each parent task to be the “external” one, multi-home it to a client-accessed project and then they’ll see just that task. But that might not be ideal.
Some likely better approaches are found in this thread containing suggestions from me and @Andrea_Mayer: