Animated parenting...?

Aside from Dan's code, there's any number of scripts on AEScripts.com that can do "dynamic parenting" and set it up for you, including DUIK, but also others.

Mylenium

This is a position expression where you can use a checkbox to turn parenting on and off. When the checkbox is on, the layer will track the "Parent" layer's movement. You could modify the first line to use it with scale or rotation as well.

p = thisComp.layer ("Parent").position;
cb = effect("Checkbox Control")("Checkbox");
accum = value-value;
if (cb.numKeys > 0){
  n = 0;
  n = cb.nearestKey(time).index;
  if (cb.key(n).time > time) n--;
  if (n > 1){
    if (cb.key(n).value) accum += p - p.valueAtTime(cb.key(n).time);
    for (i = n-1; i > 0; i--){
      if (cb.key(i).value) accum += p.valueAtTime(cb.key(i+1).time) - p.valueAtTime(cb.key(i).time);
    }
    if (cb.key(1).value) accum += p.valueAtTime(cb.key(1).time) - p.valueAtTime(0);
  }
}else{
  if (cb.value) accum = p - p.valueAtTime(0);
}
value + accum