There is not currently a way to lock the Play Store so that kids cannot download free apps. However, there are a few things you can do to restrict what they can do within the Play Store.
First, you can set up a PIN code for the Play Store. To do this, open the Play Store app and go to the Menu. Then, select "Settings" and scroll down to the "Require authentication for purchases" setting. Select "For all purchases through Google Play on this device." You will then be prompted to enter a 4-digit PIN code.
Once you have a PIN code set up, you can also restrict what types of content can be downloaded from the Play Store. To do this, go back to the Play Store settings and scroll down to the "Content filtering" setting. Here, you can choose to filter apps by content rating (e.g. Everyone, Low, Medium, High, etc.), or you can select the "Custom" option and manually select which types of content can be downloaded.
Finally, you can also set up parental controls for individual apps that you have installed on your device. To do this, go to the "User Controls" setting in the Play Store settings. Here, you can select which apps you want to enable parental controls for. Once you've done this, you can go into the parental controls settings for each app and control what types of content can be accessed.
For more information, you can check out this support page from Google: https://support.google.com/googleplay/answer/1075738?hl=en