We were there last April with DNiece and DNephew (age 7 and 8 then). It was their first WDW trip, and we do generally buy them gifts every time we see them, every time we go to Target, etc., with them, etc., and are generous at b-days and christmas (however, we normally buy them clothes & books, with makes DNiece very happy, and DNephew less so).
For this trip, we gave a set amount of spending $ for the trip, kept it separate, and kept a running tab of what each kid had bought. This should work fine for age 10, but not so well for age 3.
We did not have any problems with the "I want this" because it was clear that they were only getting that amount of spending money. If they wanted to spend it on an ice cream treat, or a ball cap or a toy gun, up to them. For the purchases which seemed to be poor ideas, we would strongly suggest they wait until another day or the end of the day and "think about it." Most of the time they decided to pass. The one time DNiece bought something against our advice, she realized pretty quickly she was not happy with it, and listened to our advice more next time.
No ideas for age 3...
We'll see how it goes this year, when the kids get a much smaller spending budget. We gave them $20/day last time because it was their first trip (first real vacation ever, in fact) but this year I think we'll limit it to $50 or $75/kid for the entire trip...