How can I selectively share app purchases among iOS devices using iCloud?
Use one apple id.

Under Settings -> "Store" on your wifes iPad and iPhone.
Set Automatic Downloads for Apps to On.

On your iPhone, under the Store settings, turn your Automatic downloads of Apps to Off.

This method will work the easiest, and will work for all situations besides your vice versa ... you purchase an app on your iPhone and it downloads to her devices.  There is no computer in the loop... her process isn't manual, it is automatic, but she can easily click and hold the icon and then delete it.