Just thought I'd ask this.
Did one of you happen to buy an app either free or paid and input the password, then another has downloaded another app within the 15 minute period in which they don't ask for the Password again?
This is set as default on the iPad.....can also be changed to immediately, so it will ask for the password for every app download.
Settings - General - Restrictions - Require Password. It will entail setting a 4 digit Passcode. Please remember this Passcode for future changes to these settings.
Another thing which could be happening too, depending on how you have all these iPads set up. If you buy an app on one iPad it may automatically download this same app to all your iPads.
Just thought I'd throw this info in.