Rebecca, over at Zenerdgy:
In the past, we had lots of money mishaps. Sometimes, they handed the money to a "responsible" parent. I usually placed the money in an envelope, which we never had handy when the child found the perfect toy at the store. Since Ben and I never remembered the amounts in the envelope, we relied on the child's memory. They always swore that they had enough money, but that often was not the case (funny, how they forget money spent but not money owed).
The rest of the post is a good, quick read on the trials and tribulations of tracking children's allowances. By the way, their app Family Bank is available on the App Store. Check it out.