My husband and I took our first trip to WDW with our then 21 month old daughter last October. We had the time of our lives and are already counting down to our return in December 2002. My daughter watches our home video of the trip at least once a day and is always asking when we get to go back to DizzyWorld.
Our trip lasted 10 days, including 4 days travel time. We did the four parks and several character meals. My daughter loved FantasyLand, especially Winnie the Pooh and the carousel. ToonTown was also great for her. Bear in the Big Blue House at MGM was a big hit. The fireworks at Epcot and MK, as well as the parades, made my daughter's face light up. Seeing her face as she saw her first character (Pluto) at Chef Mickey's on our first evening there was worth the entire trip.
My main advice is to take a nap break in the afternoons. My husband and I probably enjoyed this more than my daughter did. Tell your DSIL that her son won't be able to help but enjoy himself.