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" :)