This is about the step that needs to be taken to prevent your kids downloading adult content related apps on Google Nexus 7. Google Play Store allows users to restrict apps that can be downloaded based on their content. You can use this feature to restrict any one from downloading apps based on content rating.
Step 1: Tap the Google Play icon on your Nexus device.
Step 2: Tap the menu option available on the top right hand corner and select Settings from the menu list.
Step 3: In the Settings screen, Tap the Set or Change PIN option under User Controls.
Step 4: Enter a PIN to prevent any one changing the content filtering settings.
Step 5: Now tap the Content filtering under the User Controls and set the required content filtering setting.
The available content filtering option includes Everyone, Low Maturity, Medium Maturity, High Maturity and Show all apps. Check out this link for more about content filtering.
After making the required changes, click OK to confirm and save the changes.
The Content filtering settings can be changed by only after Unlocking the settings using the PIN.
By this way you can restrict kids from downloading apps rated as above aged.