Letting your kid play with your phone is fine and all, especially if they are tech savvy enough to download their own apps and play with them. However, with great power comes great responsibility and in-app purchases can be something of a plague. If you only own a single device that has your credit card information, you can get screwed pretty fast if something goes awry with the purchases. The easiest solution also happens to be a very effective one since you can protect your Google Play account so that it requires a password for purchases.
Go ahead and launch the Google Play store, tap on the menu and select Settings. Scroll down till you see an option that reads Password – Use password to restrict purchases and enable it.
You will be asked to enter the password of your Google account to continue so do not hesitate and do it.
Your account is now successfully protected, meaning only those that have access to your password (which should be just you) can purchase things from the store. Google does offer a limited time window after a password-enabled purchase occurs so that if you want to buy multiple products, you can do it without annoying prompts. Do keep that in mind though so that you wait for about 30 minutes before handing your phone over to your kid.