The Amazon Appstore allows you to restrict in-app purchases or disable them completely.
To enable password protection:
Settings → Parental Controls in the Personal section → Set up a password if you have not or enter one → Find Password-Protect Purchases → toggle on.
To entirely disable in-app purchases:
Amazon Appstore → Menu → Settings → In-app Purchasing → Tap 'Allow In-App Purchases' checkbox → Enter your Amazon password.
Read this article to learn more about this: https://www.amazon.com/gp/help/customer/display.html?nodeId=201357720