Since gumi's games are free-to-play, the manner of purchasing from our games is through In-App-Purchase or IAP. If you have a child and you want to restrict your app store to avoid and prevent any accidental purchases then you can check out Apple's site on how to do it. The link provided will guide you step by step on how you can safely set up your Password protection.
If you want to know how to contact Apple directly for any accidental purchases, you can read this article.
Just a reminder. Please do keep your personal password safe at all times.