I think you're on the right track but your sum is wrong, to get a child component to retain its original size:
target.scaleX = 1/parent.scaleX;
This way if parent scale is 2 (twice the normal size) the target/child scale will be 0.5 (half its original size), and in the context of its parent, its original size.
Remember that scale is a multiplier applied to an absolute dimension (width/height), not a measurement in itself - when you double the parent, you must halve the child, and vice versa, not simply add the number taken from one to the other etc
Hope this helps!