Ashley Griffiths, from St. Ives, said the boy and girl made the purchases using the password they had learned from using the iPad for schoolwork and games.
Griffith said he was unaware of the in-game buys, ranging from $4 to $121, until he received emails from Apple on Sunday detailing four pages of digital purchases.
"Children don't understand the value of money, they just see it as a way of collecting more pets and clothes for characters in the games," he told The Daily Telegraph.
"I mean, who in their right mind is going to pay £75 ($121) for a virtual pet?"
Griffiths said he was surprised how easily his children were able to make the purchases.
"They were just prompted to enter the password, and that's what they kept doing," he said.
"These games are aimed at children and the designers know exactly what's going to happen. There should be measures in place to prevent this, such as asking for credit card details."
He has since changed his password and Apple, in a goodwill gesture, has refunded the amount of the purchases, he said.