Google Play processes all in-game purchases. To get a refund for an accidental purchase, please contact Google Support directly.
Here's an article on how to request a refund for a Google Play purchase: https://plrm.me/refundandroid
If your purchase was unauthorized, please check this article on how to report unauthorized charges: http://plrm.me/392fgql
Tips to avoid Accidental Purchases: http://plrm.me/3r72YDm
Tips to avoid Underage Purchases: