As a parent, it is hard to judge another parent. Daddy could have been home alone and needed to take a crap for all I know (didn't read completely, so it might be mentioned)...
15 minutes is a short time and kids can do a lot of damage...
That said, knowing how it works, I would probably have allowe the download, then signed off my iTunes account and then give the iPad to my son. Not everyone has in depth knowledge of how iOS works.
I think Apple showed good faith, although they didn't have too. I personally would like to see a check box to indicate I want the account unlocked for a short time after entering my password.
With an iPhone, you could buy an app, then forget your phone somewhere, so this could still happen without getting "bad parenting" comments.