How to pay for my kids' apps?

I have kids, they have their Android tablets, and I have a credit card. I have created Google accounts for them, each for one tablet. I want to buy apps for them, but I don't want to enter any details of my credit card into their Google accounts, because the theory is that they will be using these accounts in the future to communicate, browse, etc. I don't want them charging my card without my knowledge. A while ago, when buying something in the Google Play Store, I was able to select between one of the tablet accounts and my own account. I then had to enter the password to my own account and was able to install the given app. Now this seems no longer to be possible. Is there a way of buying apps for my kids that is safe?