Thanks for all the great input. I forgot to mention a few things in the original posting. First, we go to Disney at least once a year, so they always have the option of saving some things for later. Second, I do make them custom shirts at home before we go, so they have never asked for a shirt or sweatshirt at the World.
Last year I took only my two oldest and while I didn't give them their own money to spend, they really didn't end up asking for a lot. Last year I purchased them each one of those little light up string things for $16 at the kiosks, a medallion necklace for like $5 at POTC for my oldest and a sword for the younger one. Other than that they asked for nothing, so I actually spent less than $30 on each of them last year.
We have been going for years though and they do have things from years past. I figured $30 was good since I invested on the pins before the trip. If you add in the cost of the pins that I bought them for the trip it is over $50 per child for the trip.