I'd probably suggest some sort of restriction on IAP amounts. Under-16s in the UK can play fruit machines, but only upto a £5 jackpot before it gets classified as adult gambling where serious amounts of money can be lost. This is on the assumption that people won't bother putting more than £5 in, as they know that they won't make a profit. This assumption doesn't hold in the IAP-world as the "profit" for the player is ostensibly, having a fun time. Although as the article shows, in reality a lot of these game monetisation tactics are really about players paying to remove an artificially enforced pain on the player, rather than to increase the fun.

So maybe just a hard limit on total IAP purchases for each game targeted at under-16s.

Also, the same restrictions that apply to gambling advertising should apply to F2P/IAP games, for the exact same reasons.

The trickiest part about it would be defining what is an acceptable game vs. an unacceptable game for the purposes of regulation. When does an acceptable F2P game like Team Fortress 2 become an unacceptable game like Candy Crush Saga? Maybe there are some people out there who think TF2 is completely unacceptable too.

I'm not saying it isn't difficult, but that doesn't mean it shouldn't be done. In my mind I see both gambling and these F2P "money" games occupying the exact same ethical arena.

EDIT: The hard cap on IAP purchases for under-16s doesn't even need to be all that low, it just needs to exist at all. Placing it at £30 would limit the potential damage done to the child, whilst also being relatively fair to the game company as it's roughly the same price as a game bought off the shelf.