Everybody makes mistakes, no matter how smart we normally are. Sometimes smart people are tired or lazy or make assumptions that nobody could be that evil, and then sometimes we pay for the mistakes. Yes, it's very nice if there's a way to recover from a bad mistake, and you absolutely should try it, but a lot of times there's nothing you can do, and you just have to live with it and learn.
I consider myself pretty smart. But if I'm stressed or tired or not concentrating, I can make poor decisions. That's how I ended up with the car I'm driving that had tons of problems. I couldn't return it, so I'm making the best out of the situation.
These parents could have been smart, and set up locks against adult content and all sorts of stuff, but if didn't know that this sort of danger even existed, they didn't consider protecting themselves against it. They should absolutely be entitled to their money back if they can get it, but at the same time it might not be possible, and it will just be another expensive lesson.
Let's say that cash-grabby DLC like this is a danger equivalent to viruses. You take precautions against it, but if you go online there's always some danger. And if you let somebody who's not prepared, like a kid, go on by themselves, the danger increases.
If the kid, while browsing, had picked up a virus that somehow bricked the phone, what would be our reaction? Probably the same.