You can use Restrictions to prevent unintentional or unauthorized purchases from your iTunes Store account or your Google Play account or your child's account. You can require a password for purchases, prevent certain types of purchases, or disable purchasing completely.

Please refer to the following links for more information:

-iOS: https://support.apple.com/en-us/105121

-Android: https://support.google.com/googleplay/answer/1075738?hl=en