If you use the steps here: https://stardewvalleywiki.com/Saves#Mobile_.28iOS.29 that should help you locate your saves on your iPhone (I recommend using the Files app rather than iTunes if you can, you may need to download the Files app first). Once you locate your saves, make sure you're moving the entire folder, it should be named something like
Name_000000000. Press and hold on that folder and use either the "Copy" or "Duplicate" options. An easy way to transfer the folder would be to then paste or move the duplicate into Downloads in your iCloud Drive (another section in Files), the folder will then automatically appear on your iPad. You'll still need to move the folder from Downloads to the Stardew Valley folder on the iPad.