Q: My grandson downloaded some games without my knowledge, using my account to pay,

Payment happens when they are bought, but you can try contacting iTunes Support and see if they will refund or credit you : 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

Which restrictions have you now set ? On Settings > General > Restrictions you can turn off in-app purchases and require you password to be entered for every download, as well as hiding the App Store ('downloading apps') and iTunes apps.