I want to update a field value from a parent task when a field value from its children task is updated.
For example, you submit 5 hours in Completed work field on ChildTask. This value has to be submitted also in ParentTask Completed Work field and if there is an existing value already, add the value, not replace it.
As far as I know, there is no such a work item "out of the box" feature to achieve it.
This feature doesn't support by both Hosted XML Process and Inheritance process.
But, in addiiton, I think you can do this with writing some custom scripts.
Workaround1:
You can do it with WebHooks.
1.Build a Api app (e.g. asp.net web api) to update work item per to parent work item through REST API.
2.Create a Webhook with Work item updated or created event and set the field filter.
3.Specify the Api app URL (step 1) and Webhook settings.
Workaround2:
Another tool named TFS Aggregator. You can set and configure apply certain rules (such as copying values from a child item to a parent item). And it will trigger after a work item is updated or created.
I know that this is very inconvenient. You may try to submit a suggestion ticket about this feature in our UserVoice website.
By the way, here is a suggestion ticket with the similar requirements. You also could vote and add comments in it.
Hope this helps.