Double clicking on a bone in the tree right now renames it. In the viewport it's free, but I'm not sure it's something you'd need to do so often to make it a double click shortcut.
I suggested something similar a while back, could this work out for you too or do you still think double clicking would be better?
ctrl+shift+arrow_down to select all children instead of just the first · #226 · EsotericSoftware/spine-editor

I've added some comments to that issue. Should we also have a way to do it with the mouse? Maybe ctrl double click selects all siblings and ctrl+shift double click selects all children? What do you think? The keyboard shortcut is a bit more powerful, since you can select all siblings, then select all children of ALL those. With the mouse your first click clears the selection, so you can only select the children of a single node.

Nate wrote

I've added some comments to that issue. Should we also have a way to do it with the mouse? Maybe ctrl double click selects all siblings and ctrl+shift double click selects all children?

Do you mean in the viewport? Since right now it does nothing, it's a good idea!

I meant in the tree, I hadn't considered the viewport. We could have ctrl and ctrl+shift double click in the viewport work the same way as the tree, that seems reasonable.