This problem has been brought up several times through the years, still I want to try to knock on the doors myself, as no apparent progress has been made, and the more we are the better after all these years.
One would expect that selecting a parent with multiple child objects would automatically select also the child objects, but this isn’t the case, and this is extremely problematic when working with large scale projects with lots of parents and child objects (which have also other child attached to them). This because, when for example we want to move a large number of parents from one collection to the other, the child objects don’t move along with the parents.
And using the select hierarchy operator in the right-click menu doesn’t solve this, as it doesn’t work with multiple hierarchies.
1-Selecting parents should automatically select its children.
2-The select hierarchy operator in the right-click menu should work with multiple hierarchies.
Please, with projects that have more than 10-20 objects it’s very difficult and frustrating to work and organize stuff without these behaviors.