We've tried handling the souvie money several different ways over the years. I agree with the folks who said to take into consideration what your kids like and how they shop. What has worked best for us is a combination of some of the things I've seen mentioned. We set an overall budget for the trip and divide that amount by the number of days we'll be there. The daily amount is all we take into the parks with us and it's generally enough to get one big thing or a few small items like pins and such. If there is money left over at the end of the day, we put it in the envelope for the next day. This arrangement worked really well for keeping DD within her budget. The only downside is that she wants to souvie shop everyday rather than waiting till the end of the trip so we tend to spend more time in the gift shops than DH would like. Sometimes even more time in there than I would like and I like to shop.
As for how much money to budget - again, that really depends on what your kids like. When DS was younger he never met a stuffed animal he didn't like. So $10 a day wouldn't have been enough. Some of DD's fave things have been the little Polly Pocket-size princess dolls that cost about $12 a set. So even for her $10 a day wouldn't have been enough. We generally try to make sure she has about $20 a day. That's a lot over the course of a 7 or 8 day trip but it's much easier to budget a little extra for souvies than to deal with her trying to decide what to buy when she doesn't have enough money to get what she REALLY wants.
As for how much money to budget - again, that really depends on what your kids like. When DS was younger he never met a stuffed animal he didn't like. So $10 a day wouldn't have been enough. Some of DD's fave things have been the little Polly Pocket-size princess dolls that cost about $12 a set. So even for her $10 a day wouldn't have been enough. We generally try to make sure she has about $20 a day. That's a lot over the course of a 7 or 8 day trip but it's much easier to budget a little extra for souvies than to deal with her trying to decide what to buy when she doesn't have enough money to get what she REALLY wants.