Our first trip by daughter was 6. She had saved about $80 to spend.
My husband came up with a great plan. We divided the money by the number of days we had in Disney. She had $20 to spend each day. If she did not spend it day one, she then had $40 to spend the next day. We promised her that if she saw a larger ticket item, we would return to get it on the last day if she still had enough money. It really made her think about the value and she wanted one nicer item rather than 5 smaller ones. The first day, she spent about $7, but then waited until the last day to spend the rest. She also realized that many of the shops had the same stuff, so we spent less time looking at the Gift Shops. It worked great for us.
We went to our bank and got a check register to keep track of what she spent. We still do that with her money today. She "deposits" and "withdraws" as needed. It is so much easier than her carrying her own cash.