Android - How to pay for my kids apps?

There's a much easier way, but it may not be supported in all countries: Buy them each a Google Play Gift Card, and attach it to their accounts. They come in different "sizes", so you can limit the amount they can pay. No identification stuff connected to the cards, and once they're used up there's no more use for them (of course, you can buy the next one).

Another positive aspect: you might decide to let your kids use them themselves. The danger is just as high as the value of the card (though content might pose a risk). So it's like "pocket money" for learning "monetary responsibility" :)

In Playstore, the user is prompted for the password (unless you disabled it) every time before a purchase can be made.

So what you can do is, make a Google Account JUST for playstore Apps that's logged in across all your and your kids Google Devices. Have every device logged in to that account on top of whatever other accounts your kids personally use on the given device.

That way you can buy whatever apps you want, but they can't since, even tho their device is logged in and has access to all the apps you purchased thru your Google Apps account, they can not purchase new ones without your password.

They can't see the details of your CC since Google will prompt for the password every time the user attempts to access Account Information.