Hi there.
I understand that your younger brother racked up on some in-app purchases for a game called Clash Royal. From what you've told us, it seems like he did make a lot of in-app purchases. I can also completely understand the need for a refund. And I'm also hearing that you'd like to know if there's a way to possibly get a refund for all of these in-app purchases rather than requesting a refund for each individual in-app purchase.
From my experience I've only made individual return requests, so I think you're right on track about being able to only request individual returns. I'm not quite too sure if there would be a way to get a refund for more than one in-app purchase, but you can definitely reach out to iTunes Support and see what they can do or can offer to help you out on this situation. If you need to reach out to them, here's where you can find them: https://support.apple.com/itunes.
Also you may or may not already be aware of this knowledge-base support article, but even if you were or weren't - I'd still like to reference it to you just in case you need it for later on: Get help with an item purchased from the iTunes Store, App Store, Mac App Store, or iBooks Store - Apple Support.
And in the future you can disable in-app purchases, so if your little brother does happen to use your phone it'll be a little more trickier to make them without being aware about it. Here's a knowledge-base support article on how to do that: https://support.apple.com/en-us/HT204396.
I hope this information helps you. And hopefully you do get your refund back, as that is quite a lot of money.
All righty.
Take care for now! 🙂
