We are going for a week in January. Our son is 12.
He is currently saving his money in a US High Interest Savings account at our local bank. He has a paper route and receives an allowance, so he is able to make weekly contributions in Canadian dollars and have them automatically converted at the current exchange rate into US dollars. Interest is paid at a small rate. The nice thing about saving this way is he knows right now how much he has saved in US currency, rather than hoarding it away in Canadian dollars and wondering what his savings will equate to once its time to leave for our vacation.
Before we go, we'll make a withdrawal of his funds. Then he's going to decide on a budget for each place he wants to spend money at (4 parks, resort gift shop, Downtown Disney, etc.). Once we arrive at our resort, we'll convert the cash into multiple gift cards ... one for each place he wishes to spend at. For example, if he saves $300 US and plans on shopping at all 4 parks, the resort and DTD, then we'd get 6 cards worth $50 each. Then, when we leave for the park in the morning, he takes one of his cards with him. The rest can stay in the room safe.
This will help him to budget, and take away any chance that he'll be overzealous in his spending and end up broke before the vacation ends. Plus, taking only a portion of his funds with us each day is safer than carrying a lot of cash. And using a gift card keeps everything easy.....no change to make, just scan and go.