If you sign out of iCloud or disable the syncing feature for a particular iCloud service first, what you do on that device does not transmit to other devices using iCloud. So disable photo syncing on the iPad and then delete the photos and they should remain in your iCloud on Apple's servers as well as your iPhone.
Read this support article about how to change your iCloud feature settings [iOS, Mac, PC] - Change which apps sync and store data with iCloud - Apple Support - "'When you turn off an iCloud feature on a device, information stored only in iCloud for that feature is no longer available on the device. If you turn off Mail, Contacts, Calendars, Notes, Reminders, or iCloud Photos on just one device, you can still view and change your information on iCloud.com and on another device you set up for that feature."
Of course if you then enable photo syncing on the iPad again, all the photos will eventually repopulate on your iPad.