How can you tell if your kid has hidden apps on there ipod

Hide and unhide apps [iOS and Mac] for your iPhone, iPad, iPod touch, or Mac - http://support.apple.com/HT201322

You could kind of hide them if you didn't mind deleting them and re-downloading them when needed.

You can hide purchases on iCloud but not on devices.

If you don't want kids to download apps set restrictions

in Settings > General > Restrictions > Installing Apps : OFF

Note you have to enable restrictions with a passcode to make changes (don't forget the passcode)

Technically, it is not hidden if you don't have it on the device.

Yes you can delete and re-download them at any time, but once on the device it cannot be hidden.

If the app is in the device, Settings -> General -> Storage should show you all the apps on the device.

If the app is being used recently, Settings -> Battery should lists its power usage