My DD10 gets plenty of money throughout the year from her over-indulgent aunt, so I don't give her any. But she is good about saving it, and as a result should be heading to
Disneyland in a couple of weeks with about $80. I don't expect her to spend all of that, since we've done either DL or WDW every year for the last 5 and she's past the "gotta have it all" part (probably because we already have it all from previous trips!

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I would say about $50 is a good number for kids of that age on one of their first visits to have for spending money. That's enough for a tshirt (or a purse or bag in my daughter's case), plus a few smaller trinkets. Some people would probably say that's not enough, but I don't think kids have to buy everything in the parks. I've found one or two items are usually more loved and appreciated than loads and loads of stuff. And I don't include snacks in the $50 - the rule in our house is that she cannot spend her money on junk food, Mom gets to decide when and where that will be bought and pays for it.
One thing I do when we first hit the parks and my daughter gets a case of the "I wants" is to remind her that we are there for x days and she hasn't had a chance to see everything yet so she might miss out on something she really wants later if she has no money left because she spent it all right away. I have her keep a notebook and write down what she saw and where. Then sometime towards the end of the trip we'll evaluate what items she really wants, and we'll go back and get them. Since a lot of the stuff is available at every shop on every corner, that works well. I have been burned once or twice, however, when she wanted the 1 item that is only in 1 store in 1 park that I had no plans to return to!