I would like to be able to find all issues, which are children of Epics meeting certain criteria, e.g. all children issues of Epics in project X (or any other JQL condition.

Something like this:

issue in parent("key = value")

e.g.

issue in parent(project = XY)

Another example would be like:

issue "key" = "value" AND parent ("key" = "value")

e.g.

issue in status = done AND parent (status = in progress)

Currently, this is not possible without a specialized app.

Thanks,
Daniel