This year, the European Union’s Consumer Protection Cooperation network, the EU Justice Minister, and the UK’s Office of Fair Trading have all expressed concerns about consumers being confused or misled about in-app spending; particularly on freemium games, and games aimed at children.
Their recommendations include developers providing better information about the true costs involved in freemium games, and ensuring that children are not exhorted to buy in-app items or persuade an adult to buy items for them. These are a good start but time will tell whether they are effective.
In related gaming (that is, gambling) news, as of this month gamblers in England and Wales will be able to set limits on the amount of time and money spent on high-stakes gaming machines (e.g. slot machines) in betting shops. According to BBC News, there are 33,000 fixed-odds betting terminals across England and Wales, on which approximately £40 billion is gambled and £1.5 billion lost each year. These terminals will now provide alerts to gamblers every 30 minutes or £250 spent. Despite these moves, the UK government said that more could be done, so clearly this is not the end of the road for gambling regulation.
There is a very, very big difference between gambling and (some) freemium games. Freemium games are not even in the same ballpark when it comes to harm against society. However, they also have a few things in common, most notably their use of behavioural psychology and compulsion loops to keep players playing more and spending more. There are plenty of freemium game players who will spend hundreds of hours and many pounds playing them, and then regret their actions afterwards – I know because I was one of them.
What would freemium games look like if they adopted the same kind of limits that fixed-odds betting terminals will have? Here’s a possibility I mocked up:
Feel free to repost these images but please link back. These are mockups. Any relation to existing apps is intentional but meant only for comic effect. I welcome any corrections.
mssv 
I really like this idea.
The only thing I’d make different is that I wouldn’t manage the spending from within the app itself, but only from the general settings.
What I mean is: I’d manage this as an OS thing, and not something the devs have to implement in-game (though not breaking current apps).
In fact, it could be put under the “restrictions” menu (where the “Disable in-app purchases currently is).
But I agree it would be better to make it more prominent, creating a new item in preferences (as in your second image).
(Sorry for the double comment)
To make it more clear: the problem with managing the limits as in your first image is that is would be pretty useless with kids. They could set the higher limit (“$100” and “Unlimited”) when the popup appears.
If it’s something you manage globally (specially in Restrictions, where you have to enter a code to change settings) it would be more secure, and you will be sure that your kids are not going to be fiddling with spending limits.
This spending limit clearly needs to be controlled by the owner of the device.