Could you please advise how I can transit only first sub-task on parent issue transition.
It will require JIRA Workflow Toolbox 2.1.28 or higher. It can be done using 3 post-functions:
Copy a parsed text to a field post-function to write the name of a transition into virtual field "Ephemeral string 1" virtual field. In this example I use transition "Start Progress":
Copy a parsed text to a field post-function to write the issue key of first sub-task order by a certain field into virtual field "Ephemeral string 2". In this example I order sub-tasks by creation date-time in ascending order:
Text to be parsed is: toString(first(sort(subtasks(), {00009}, ASC)))
Note that:
Update issue fields post-function for copying the name of the transition in "Ephemeral field 1" into virtual field "Execute transition" of issue whose key is in "Ephemeral field 2":
Text to be parsed is: issuekey in ("%{00062}")
Note that:
Once transition is configured, it will look like this: