my kids purchased 100$ for game coins by mistake how i cancel the purchase and Refund ??

Purchases are considered final but you can try the 'report a problem' page to contact iTunes Support and see if they will refund or credit you : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

To try and stop it happening again you can use Settings > General > Restrictions on the device to prevent in-app purchases.