To an extent, this is now possible.
Go to the project Settings -General tab, section Sub-issues: Here you have several options for how parents and sub-issues should behave with respect to each other:
Especially the last three are of interest here:
- Close parent when all sub-issues are closed: Check this to automatically close the parent issue when all its subissues are closed.
- Set parent status to X when sub-issue is in progress: Check this to automatically set the parent status to the defined status (Default is Work in progress) when one of its sub-issues is set to "In progress". This is typically used when you have set up work to be done as parent + sub-issues, so that the parent is in-progress when work has started on one sub-issue. The statuses available to choose are the ones with the In-progress flag.
- Close sub-issues when parent is closed: Check this to automatically close all open subissues when the parent is closed.
Control in what order sub-issues should be completed
Under the tab "Sub-issues", on the parent issue, you can select a check-box "Sub issues should be done in order". If this is selected the sub-issue on the top of the list needs to be completed before you can work on and change the status of the second sub-issue and so on.