How enable monetary parental control for Google Play ? The last Google Play update removed the PIN option.

The new solution is email login.
My kids know, and should know, their email logins,
so that is not protecting me from Google Play purchases.

How to enable monetary parental control for Google Play ?  

I'm only interested in monetary protection
(content protection is another topic).
The easiest solution is to not have a credit card linked to their account. Even if you did still have a PIN for protection, they could still use the card online with any place that takes google wallet. The PIN (and now password) really only protects purchases from the device itself.

There are a few options that come to mind for them to have paid apps:

Second Google Account

One option is to add your Google account to their phone, and use it for all the purchased apps. When they want to buy an app, they switch over to your account, and then you put in your password to buy it. The downside of this method is that they could easily change the sync settings so they get your contacts, email, etc.

Temporary Second Google Account

Alternatively, you could temporarily add your google account, make the purchase, then remove the account. The purchased app will remain on the device and should work, and they won't have access to sync data. But the app won't get any updates, and you'll have to do the same process any time they want another app.

Third Google Account

Or you could create a third google account that only is used for purchased apps, and not for emails or contacts. They would still need your password to buy the app, and you wouldn't have to worry about them activating sync for your information. And since the account would remain on their phone, their paid apps could be updated as well.

Prepaid/Gift Card

Another option is to put a prepaid card, or a Google Play Gift Card (min $10), on their account. They can purchase up to the limit of the card, either for paid apps or in-app purchases.

Alternative App Stores

You could also try using a different app store, such as Amazon's app store. It has most of the apps that Google Play Store has. It doesn't have PIN/password protection for paid apps (but it does have that option for in-app purchases), but Amazon gift cards can be bought in denominations as low as 15 cents. You could just buy the specific amount to cover the app they want, then gift it to them so they can buy the app.