Hi Linda,
Congratulations on your upcoming FIRST trip to Walt Disney World!
Yes, snacks and water can be brought into the parks. Glass containers are not permitted with the exception of baby food jars.
Coolers have size restrictions.
Spending money is very dependent on your budget, touring style, plans for meals, etc. Some parents restrict their kids to spending a certain amount all week and others allow an extra treat each day. There are families that can tour the park without buying a balloon, light-up toy at parade time, popcorn before the fireworks, and don't mind leaving without a set of Mickey Ears.
There are other families that have very high souvenir budgets.
Will all of your snacks be brought with you? Or will you stop for a Dole Whip, Funnel Cake, or Root Beer Float?
I would take a look at some menus at www.AllEars.Net to figure out your meal budget. Then, you can decide what you are willing to budget for the little extras.
Have fun and make magical memories!