Can you mask a parent null object in Adobe After effects?
Well, there a lot of ways to get result quicker, but each one depends on desirable result. =)

Here's one way:


Make desired null's mask
Make any mask for it's child (it doesn't matter what there)
Alt+click on child's mask path 'clock' icon (it'll open script settings)
Clear edit field or select all the contents
Drag 'link' icon (spiral) of mask path expression over Null's mask Path (it should add some kind of "thisComp.layer("Null 1").mask("Mask 1").maskPath" to expression field)
Copy this new mask to all childs


p.s.: this way is just a start, since it's not includes shifts of child and parent layers, but if pivots aligned it'll work