By following coding suggestions provided by "patrickmj" here
- (thanks Patrick!!) and modifying the code a bit, I managed to get something like that in my Collection_Tree page.
Ignore the Greek, the image shows a parent collection followed by 4 sub-collections, followed respectively by a number which is the "total_items" in each collection.
My question: How can I get the parent collection number to show the SUM of total_items in sub-collections, in this case, number seven (7)??