The following is how to restrict in-app purchases on Android.

Require password entry for in-app purchases

  1. Open the Google Play Store application.
  2. [Menu] > Tap [Settings] > [Authentication ] > [Require authentication for purchases].
  3. Select the [For all purchases through Google Play on this device] option.

Set parental restrictions in Google Play

Parental Restrictions can be applied to an Android device to limit the content that can be downloaded or purchased from Google Play on that device, depending on the age range for which the content is intended.
For more information, please visit Google Support Page.