By default the App Store and iTunes only requires a password every fifteen minutes or so. That means if you've authorized a single purchase, someone could technically purchase something else without being required to enter a password again. For parents who want to prevent this from happening, you can easily use restrictions in order to require a password be entered for purchases immediately and every single time.
How to use restrictions to enable an iTunes and App Store password for every purchase
Launch the Settings app on your iPhone or iPad.
Tap on General.
Tap on Restrictions.
Tap on Enable Restrictions if they aren't already enabled. If they are, skip to step 6.
Pick a password that only you will know. It should be different than the Lock screen passcode and your child should never know it.
Tap on Require Password under the Allowed Content section.
Tap on Immediately.
That's it. Just close out of restrictions and you're good to go. Now anytime someone goes to purchase content from iTunes, iBooks, or the App Store, they'll always be prompted to enter a password. That means if they don't know it, they'll have to ask.