A simple expression created by the picwhip to link the properties is all that is needed.
I had trouble understanding your answer, but I got a response elsewhere that explained it. For anyone wondering you just Alt+Click or Opt+Click the keyframe button of the parameter you want to parent. The pick whip button will appear below (next to a few other buttons like an equals sign/graph/arrow) and you can drag it to the property you want it to follow on the parent object.
Follow up question,
I dragged a pick whip from one shape layer property to another (scale). I now have one box following the scale properties of another box (great!) but they scale in different directions... I tried aligning their anchor points which mostly works, but this is odd to me because the when I make a parent/child relationship with overall layers, they follow the position and scale of the parent's anchor point, regardless of where the child's anchor point is.
I tried to make a pick whip expression of the child object following the parent's anchor point to see if this would work as a quick fix, but now it flies off the screen.
Any advice appreciated.
Pick whipping properties in expressions only makes the value change for that one property. It is very unlike parenting despite them both using the pick whip to choose them! You could make the rotation of one thing control the scale of another (or get even more complicated than that).
If you try to tie properties to other properties when there are parent/child relationships involved, things can get really nuts. Without seeing more of your project, it would be hard to give exact advice.
I need it to retain it's stroke width constantly throughout the animation. If someone has a solve for that, maybe I could work around it entirely.
depends on your setup, but adding layer style - stroke will render after transformations and will give you constant stroke as the shapes grow. another workflow might be using the repeater operator but that also depends on your setup. try showing us more of it so we can better asisst you.