Welcome to Apple Support Communities. I understand that you'd like to prevent the photos from your iPhone from appearing on other iPad devices in your household. I know it's important to be able to separate your data and I'd like to help. To do what you'd like, there are a few options; if you'd like to keep the Apple ID the same on all devices, you'll want to disable iCloud Photos on the devices that you don't want photos to appear on. Keep in mind that this will also prohibit photos that are taken on those devices from being uploaded to iCloud Photo Library or your iPhone.
You'll also want to check that "Upload to my Photo Stream" is disabled in the same menu on both devices.How do I turn off iCloud Photo Library on just one device?
You can turn off iCloud Photo Library on a single device when you follow these steps:
On your iPhone, iPad, or iPod touch:
If you have Optimize Storage turned on, you might not have all of your original photos and videos on your device. To download the original photos and videos on your iOS device, go to Settings > [your name] > iCloud > Photos and select Download and Keep Originals. On your Mac, open Photos, choose Photos > Preferences, then select Download Originals to this Mac. Then you can turn iCloud Photo Library off. You can also select the photos and videos that you want to download from iCloud.com.
- If you're using iOS 10.3 or later, go to Settings > [your name] > iCloud > Photos, then turn off iCloud Photo Library.
- If you're using iOS 10.2 or earlier, go to Settings > iCloud > Photos and turn off iCloud Photo Library.
Get help with your iCloud Photo LibraryIf you have both iCloud Photo Library and My Photo Stream turned on for your device, any new photos you add to your library or take with your iOS device can be seen in the My Photo Stream album on devices that don't have iCloud Photo Library turned on.
You can also look into enabling a Family Sharing account where you can set up an Apple ID for each child, but still monitor and share purchases and other content. You can learn more about Family Sharing here: Family Sharing
Best Regards.