Hi Alwyn
What you ask is both highly sensible and difficult to achieve!
A crucial question is whether your wife has "iCloud Photo Library" enabled on her iPad. (found in Settings > Photos & Camera > iCloud Photo Library).
If Yes, then things are tricky. The recommended way is to disable this function, but that, too, brings problems (see links below).
If No, then the situation is easier. Open Photos on the iPad then:
Option 1. Click on Camera roll which should show all your photos. Choose Select and click on every photo you want to delete - the Trash can icon should be visible. Click that to delete. Even then, it will put them in the Recently Deleted Album in case you change your mind.
Option 2. Click on Albums. Choose an Album > Select > Delete
Now, if you have My Photo Stream enabled, you can delete from there in the same way, but it will delete from all your iOS devices, but leave the photos on your Mac (in Photos presumably unless you're using the older iPhoto). If My Photo Stream is not enabled, you can ignore this bit.
**
If after trying Option 1 or 2, and Selecting a photo(s), the Trash icon does NOT show, this almost certainly means that these photos were placed on the iPad via sync with iTunes.**
Attach iPad via USB to the Mac. Click on the iPad icon and from the drop down, choose Photos. Click on the Sync box. Then Uncheck the Sync box. A message will come up saying "iTunes will no longer sync photos to the iPad - Do you want to keep or delete photos previously synced to your iPad?" Underneath are two boxes "Keep Photos" and "Remove Photos". The choices are self explanatory and "Removing Photos" is from the iPad only.
Sorry it's so long-winded, but it's a really difficult issue to sort out.
** I think this is the option that applies to you**.
These links are for info - not enormously helpful; more often stress the difficulties!
https://www.quora.com/How-can-I-delete-photos-from-my-iPhone-but-keep-them-in-iCloud
https://discussions.apple.com/thread/7596277?start=0&tstart=0
https://www.lifewire.com/delete-photos-from-ipad-1994634
Ian