I just feel this whole edit tags screen is very cumbersome and useless. When you have the tags displayed on screen you already have everything you need implement the maintenance. This is .NET framework and the treeview is very poweful. The way I see it things would work like this.
No "Edit tags" screen, everything is done from the regular tags list through a context menu from a right click. I would also have drag & drop support to move tags easily. And would never be so lazy as to display a popup message saying "You must remove all the child before you can remove the tag". I would tell the user that all child tags will be deleted, ask for confirmation and proceed with deleting them upon confirmation.