We need people to start owning up for their own mistakes. The password was put in by an adult, he's responsible for that purchase.
To an extent. Even the best parents in the world can't be attentive to every single thing their child does 24 hours a day, 7 days a week, 365 days a year. All it takes is one tiny slip up for something like this to happen.
"Hey dad, can you put in your password? I want play this free game while I borrow your iPad"
"Yeah, sure thing, son".

uts in password, hands iPad back to kid:
"Thanks, dad! Gee wizz, you're the greatest"!
:kid goes into living room, fires up the app, and starts hammering the buy 30 coins for 99 bucks button during the 15 minutes the password is up in the App Store:
See? It's a fairly simple thing to happen. Admittedly there are the parental locks inside of iOS, but not everyone knows about them, and most people won't even think to activate them if they only let their kid use their own iPad occasionally.
So who's to blame? If it only happens once, then it's just one of those things. You can't blame the parents for not knowing what their kid was intending to do here. Can't blame Apple for allowing it to happen. And the developers? They've gotta make money. It's not really their fault a kid goes haywire with in app purchases. Just consider it a lesson learned. Kid gets scolded. Apple refunds the cash. Life goes on with everyone a little wiser.
If it happens again and again and again, then yeah, go ahead. Blame the parents. Once is understandable. Twice is even excusable if it happens like, months later when they drop their guard (this AGAIN). But more than that, and it's their fault for not locking the iPad down when they hand it over to their kids. They know what could happen.