How old are the kids?
For our kids, who are now 13 (nearly 14), 10, 10, we sort of let them make their own decisions with their money. They earn money through allowance, chores, their b-day, Xmas, etc... They have a bank account that DW and I put some of that money in, but they also each of their own wallet. Certainly we have to approve any purchases, but we let them make their own decisions how to spend it. We remind them often that they should be saving "Disney money", and that thought process goes in for everything they do. They all actually have a separate envelope they keep "Disney money" in. So when we go to WDW, whatever they're saved is whatever they have to spend. Through our repeated discussions with them, they're all pretty smart about it. DS10 especially, he's cheap, just like his father.
They all have several hundred dollars when we go down. However, they never wind up spending it all. Even when we're down there, while we let them buy things, we try to talk them through the "do you
really want this or is it just an impulse decision" process. They do quite well with it. And sure, we do let them buy something on an impulse from time to time, but it's usually a small trinket, nothing "big". I'd say our kids spend about $75 each during a 2 week trip, maybe $100 if they feel like splurging.
If this is an infrequent trip, and you've got the financial resources, I'd have no problem with them having $100 each, or even $200. Things in WDW are quite expensive. A sweatshirt can be $50...easily. I'd just be sure to walk them through the process of making good decisions on what to buy, not just buying something because it's burning a hole in their pocket.
Frankly, as I mentioned earlier, I'm quite cheap. We do all kinds of things to save money on our trips...stay offsite (we would do it even if it weren't a money issue, but the money is nice), pack our own food, etc... When we're there, we don't buy experiences, etc... On a 2 week trip, I'd guess DW and I spend less than $50 total on WDW stuff (snacks for the kids or us not included...even that isn't much). However, having said that, this summer is the last trip we have on our AP and we don't plan to renew. I'm sure we'll be back to WDW, but it may be a few years. I've been squirreling money away since last summer...I plan to head down with $1000 minimum in spending money and just do things we normally wouldn't do, or buy things we normally wouldn't. Certainly not going to go hog wild...but splurge more than we normally do.