Smartphones have capability to lock their respective stores with a password. Parents should make use of that and not give their credit or debit card information to minors too young to understand value of money. In fact, people should never give credit or debit card information to anyone, including their spouses and children. Granted, not even all adults understand value of money but that's different topic. As far as I know, Visa Electron is the card of choice for minors as it can be granted to as young as 10 years old. But that card is their own and parents can supervise its use and set daily and monthly withdrawal limits.
The bottom line is that parents more often than not, have means to supervise their minors use of money. They should exercise that and not blame outside parties for their own ignorance. While parents should actively seek out means to protect their minors in this brave new electric world, I have to admit that not nearly enough is done to educate parents that such things can happen in the first place and how to prevent them.
That said, there are companies out there that are blatantly exploiting minors and ignorant parents alike. It's not moral, but is is legal. Companies have throughout ages sook to exploit people to gain profit, this is nothing new. Only the means to do so are. Well, relatively new as they have existed since smartphones became popular.