I recently answered a reader’s question about whether deleting an image or video in Photos for OS X and iOS with iCloud Photo Library meant the photo was gone forever. (The answer is: Not immediately unless someone then also deletes it from the Recently Deleted special album.)
I said there was no way to prevent someone using your iOS or OS X device from using the delete feature, but Ralph van Doorn wrote in with an excellent suggestion!
I would recommend that you set the iPhone or iPad on Guided Access and disable the delete button. You set it once, and whenever I give my iPad to my son, I start Photos, and triple-click the home button to start Guided Access, and I don’t have to worry about photos being deleted.