Disney is bigger than you probably expect. Four parks - and that doesn't include the non-Disney parks (SeaWorld, Universal, etc).
Have some sort of plan before you arrive at each park. The parks are big, so make yourself familiar with the layout and location of the attractions you are most interested in. There will be plenty for a 2 and 5 year old. You don't need to plan every minute - but having some idea of what is where, and when to ride the most popular attactions helps.
An hour first thing in the morning is worth three in the mid-afternoon. A Disney vacation is a wonderful time to take naps -- but generally not so good a time to sleep in.
Book a character breakfast. They are pricey, but your best chance to interact with characters. The restaurant board has several threads at any given time on which character breakfast is best.
Considering making Priority Seating arrangments at a sit down restuarant every other day or so. Although its possible to get variety from counter service - and even possible to walk into sit down restaurants and eat - adults can only consume so many chicken fingers from counter service before going insane.