My kid spent 40 dollars on brawl stars and I want a refund

You can try the following: Request a refund for apps or content that you bought from Apple - Apple Support

Also see excerpt below from that link:

If you or a family member bought an item by mistake, set up Screen Time to prevent unintentional purchases in the future. You can require a password for purchases, prevent some types of purchases, or disable purchasing completely. Set up Screen Time.