I can't speak for data ownership for the others but at least Google has its Inactive Account Manager [google.com] which lets you "will" your data to someone else. You can also, at any time, download all your photos, video, and other content using Google Takeout [google.com].
Which doesn't help you if you don't have the username and password. If the sons had the Apple Id and Password, there would be no problem. Let me summarise what actually happens:
1. If you use a passcode (4 digit PIN or more) and forget it, nobody can access the data on your device anymore. However, you may be able to reset the device.
2. If you haven't turned no theft protection ("Find my iDevice") you can always wipe the device, So you just have a new, empty device. If "Find my iDevice" is turned on then you need the AppleId + Password to wipe the device.
3. If you wiped the device, have AppleId + Password, and used the free iCloud backup, then all the data can be restored from the cloud.
It seems that "Find my iPad" was turned on and they don't have AppleId + Password.