depends on ages of the kids...IMHO...young kids will want a lot of the stuff they see,it helps to just tell them they can pick one item say,on the last day,or whatever,or dollar tree disney stuff usually suffices for little kids,who like little silly toys,lightsticks,etc.
Older kids start wanting more expensive items, and buying junk ahead of time doesn't work so well.... I always had my kids save their money for their toys,etc they were going to want-(from allowances and jobs) and I would buy them one thing per trip,usually a t-shirt,or a hat,or a stuffie (again depends on age) one of our fave souvies was the year I bought each person in the family a park specialty coffee cup.....(not generic that you can get at walgreens) we all still have our cups many years later.... My one rule was for the gift I would buy,it had to be unique,no random stuff that you could get anywhere,but something interesting and specific to their choice. so when they were younger,about 20 bucks per kid on one item,and the rest was up to how much they wanted to save and bring.