Stop syncing photos between devices & delete synced photos

I purchased an iPad for my child for school, & have the device under my Apple ID. However, all of our photos are now syncing between devices. The advice I’m seeing is to turn off “iCloud Photos” on both devices. When I tried, however, and we went to download the photos to our devices, it indicated they would take up too much room, & we’re thinking it’s because it’s trying to download all of the photos from both devices to our iPad & iPhone. Is there a way to unsync our photos so that we can save only the photos we’ve taken to our devices? All of the information I can find to do this requires connecting our devices to a computer & going through iTunes, & we only have the iPad & iPhone, no central computer.