Follow these steps to setup restriction on for your Google Play Store account.
1. Open your Play Store and go to the Settings Menu.
2. On the Play Store Settings, you will see the option "Require authentication for purchases". This option prompts you for a password before purchasing anything using your account. You can set it by the selection below.
3. After choosing the setting you want, you will be then asked for your password then you're done!
Be safe and keep your password to yourself to ensure the security of your account.