Depending on how old they are...
We personally will have a specific amount for the kids to spend TOTAL for the whole trip. When its' gone, it's gone. We always to Downtown Disney on the day we leave so we try to get them to wait until then...What I always do is ebay or like right now, the disneydirect sale that's going on, stock up, but keep the things hidden so that you can give them one or 2 things per day while you're on your vacation.
Or when you get there, figure out how much for each child, get them both that amount in Disney Dollars and let them spend it however they want.
Like I said though - it all depends on how old they are.