Hi bartvanparys,
It's not currently possible to group tasks by their parent task, or otherwise to view subtasks in a grouped view under their parent task.

You can vote for this idea in our Ideas forum if you'd like.

Let us know if we can help with anything else!

FYI, a workaround might be:

Smartlist:
list:work AND notContains:[]

I use the Note for my task:
[] Do something
Notes about my task...

Then when it's done I either close it, or put an x: [x]. This hides it from the smartlist.

Not necessarily ideal, but thought I'd throw it out there because it solves my use case.

@cobrastyle
Neat

Typo: should have been:

list:work AND noteContains:[]

GOOD:
I can have as many 'subtasks' as I want in the note.
I can have notes about the task readily visible.

BAD:
Can't rearrange notes in case you want to do that.

PS: Thanks @martingchapman