3.5 is a great age for WDW! We generally go for 7 or 8 nights...usually Sat-Sat, or Sat-the next Sun. We have done this since DS was 2.5 he is now 10, and we have DD who is now 3 yrs old. We once did a 10 night trip, but DS was 5 then (before DD was born). It was a wonderful trip.
I like going for a week because there really is a lot to do at WDW, I have been over 30 times personally and I have never done everything. We like to have 2 days at MK, 2 days at Epcot, 1 day at MGM and one day at AK and then an extra do pick up what we missed, swim or do water park, or shop. We take mid-day breaks or we will stay late for EMH and sleep in the next day and go in after lunch, depending on the park of course.