Unparent the null place it at the top then paste this expression on the position of all layers underneath–must be in order in the timeline since this expression uses the relative (index-1)–in other words it pulls info from layer above). This one only reads the y value if that’s all you want to affect:
[value[0],thisComp.layer(index-1).transform.position.valueAtTime(time-2)[1]]
Or if you want to have control over both values:
[value[0],value[1] + thisComp.layer(index-1).transform.position.valueAtTime(time-2)[1]]