The coins are a "fun" value add thing. You can get small discounts on digital games if you use the gold ones. The silver ones can get you various digital/physical goodies. You're not missing out if you don't use them.
This is not fun! This is straightforward psychological manipulation! It’s taking people’s loss aversion [1] and grinding it as hard as possible for as much profit as they can. This is especially bad when you consider that they are a family-oriented company that primarily markets to children.
So for example if you acquired 10 gold coins during July 2020 (it doesn't matter when or how many purchases or how spread they are through the month), all ten of those coins would expire simultaneously at the end of July 2021, while any coins you acquired later would remain. It's not a radioactive decay at all; it's a simple expiration date on each individual coin, like you get on coupons or other incentive programs.
The coins expire for the same reason that coupons expire, airline loyalty program miles expire, etc -- they're a liability on the company's balance sheet which potentially could build up infinitely if there wasn't some limit to how long they needed to be tracked and redeemable.
Regarding e-shop: I'd never let my hypothetical children have access to any kind of e-shop where you can purchase without me having to confirm. Parental controls FTW.
The alternative is to uncheck “save payment method for future purchases”.